First of all, this is entirely subjective and I realise some people consider it alarming that anyone would like to own a machine that isn't as nature/time dictated - this post isn't for you 
I'd say that a collector is after perfectly functional, mint or near mint tables. Selected improvements that make the table shiny but are easily removed/replaced without permanent modification necessary a bonus.
A tinkerer is an enthusiast who enjoys improving and modifying machines.
A player is someone who just lives to play, they care that the machine plays well and smoothly and if they are interested in a mod, it's something that affects gameplay and all aspects of it first and foremost.
I must add that I'm a collector/tinkerer/player in probably equal parts. I love to fiddle, tinker and improve things, learning a ton along the way, when I'm happy with a machine then I tend to put the time in playing it, but largely not before I enjoy seeing others enjoy the tables too.
I take a lot of pride, probably too much (!) in making my machines look, sound and feel as good as they can be. I really try and make things shine in every respect so here's a guide to modding and improving your machine in my experience - likely more useful for people new to the religion.
Anyway, here's various mods and my opinions, in no particular order. Apologies if you don't agree or if I've missed a mod/upgrade, thats what the comments are for.
PLAYFIELD
Invisiglass/PDI glass
Cost 2/5, Value 2/5, Aesthetic Improvement 3/5, Modernising 0/5, Skill Req. 0/5
An easy improvement and looks lots better, pity its generally quite expensive. I install it on all my machines though. Not always easy to find in the UK.
LEDs
Cost 1/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 3/5, Skill Req. 2/5
I take bulbout of all my machines and replace them with good leds (comet/cointaker 2/4 smds). I don't care so much about the back-box. I think that doing them yourself rather than ordering kits allows you to experiment with how a table can look and this can be great or atrocious. The advantage of less power and long life live alongside ghosting, flickering - although this is much less reduced by used Led OCD. There are many great and exhaustive led guides elsewhere
Playfield Spotlights
Cost 1/5, Value 4/5, Aesthetic Improvement 3/5, Modernising 1/5, Skill Req. 3/5
Cheap and great for spotlighting dark areas of your table. You can buy ready made mods or build them yourself. Depends on your location lighting.
Wire Lights
Cost 1/5, Value 2/5, Aesthetic Improvement 2/5, Modernising 1/5, Skill Req. 3/5
In my experience I've found these fiddly and need for the table to be in a dark room. Not for me and I just don't think it adds that much, but would work on something futuristic like Tron.
Mylar covering
Cost 1/5, Value 3/5, Aesthetic Improvement 1/5, Modernising 0/5, Skill Req. 1/5
You can use this covering in patches (large or small) to counter table wear. I've not felt the need to add mylar to my own tables.
Convulux/Plastic Protectors
Cost 1/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 2/5, Skill Req. 2/5
A recent addition to my tables that add aesthetic value and protection to common plastics, which can be expensive and/or difficult to source if they need replacing. I really like the yellow, red and green neon - but I was disappointed with the blue ones. I've found also it's better around superbright wide leds. I love them on my AC/DC (yellow) and AFM (yellow). Red looks good on TWD and MMR. I'd probably avoid the sets that just comprise of 3-4 pieces compared to 7 and up on some Sterns. Big sets are quite pricey and you need to be comfortable with removing screws, ramps and stuff to fit them.
Cliffys/Protectors
Cost 2/5, Value 3/5, Aesthetic Improvement 1/5, Modernising 1/5, Skill Req. 3/5
Largely speaking look at protecting ramps and common plastics that take a battering. Also drop holes and chutes. A plastic is generally easier to replace than the playfield. Can be fiddly and again you need confidence removing playfield parts in some cases.
MODS & TOYS
Cost 2/5, Value 3/5, Aesthetic Improvement 1/5, Modernising 1/5, Skill Req. 3/5
I've got a lot on my machine and its worth using the web for recommended ones. Avoid cheap tat (although I like the UV flashing backbox strips that are triggered by flashers) and the right mod with lighting/animation can add a great deal. If you're unsure if a mod will look good, then ask 2-3 other players, there's a lot of very poor mods out there. I know I bought plenty of them when I was somewhat more naive. They can also cost top dollar, so shop around. I went mad with TWD LE, because there's some great mods for it.
Replacement playfield
Cost 5/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 5/5
Difficult to get the one you're after and there's a fair price to pay, not only for the board for the restoration thats likely to follow.
Mirror/Decal blades
Cost 2/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 2/5
A fairly easy add to most playfields and can mean an instant improvement as mirror blades reflect lights and flashers, or themed insert blades make the game look very cohesive. If you have them fitted, be wary lifting the playfield and scratching them. You will swear. A lot.
Aprons
Cost 4/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 2/5
Options for new overlays are good, even if not many tables are covered. I've got a couple of more expensive aprons (metal) and even illuminated ones. Look good but likely that most will just go for nicely printed game cards for their original apron.
BACK BOX/CABINET
LED OCD
Cost 2/5, Value 2/5, Aesthetic Improvement 3/5, Modernising 1/5, Skill Req. 3/5
It's funny that you don't notice the effect this card has until you take it off. A pretty remarkable PCB that smooths your LED inserts and giving them back the glowing/pulsing qualities of original glass bulbs. One of those once you have it, you need it in all. However it's pretty expensive for what it is and players are unlikely to notice, hence the OCD name I guess. Note that only inserts are affected, to do GI (general illumination) the board below is required in addition. You need to connect to a pc to make the most of it and the software is quite flakey to get to run smoothly.
LED OCD (GI)
Speaker Upgrades
Cost 3/5, Value 2/5, Aesthetic Improvement 1/5, Modernising 1/5, Skill Req. 2/5
Popular upgrades that aren't cheap and to be honest the "upgrade" isn't all that, save for generally controlling the balance mix between back box and cabinet. I've upgraded a few machines but always been somewhat disappointed with the actual improvement.
Speaker Lights
Cost 1/5, Value 3/5, Aesthetic Improvement 3/5, Modernising 2/5, Skill Req. 2/5
I've got a couple to fit, but the fact they were at the back end of my purchasing says a lot about them. It adds visually I guess, but money is better spent elsewhere.
Under Cabinet lighting
Cost 1/5, Value 3/5, Aesthetic Improvement 2/5, Modernising 2/5, Skill Req, 2/5
Relatively cheap addition but I can't fathom how it would look in machines close together and using different colours. Not for me.
Toppers
Cost 3/5, Value 3/5, Aesthetic Improvement 2/5, Modernising 2/5, Skill Req, 2/5
Can add value and interest to a machine, but I think are expensive. Some machines need the original topper to maintain value.
Custom rails/armour
Cost 2/5, Value 3/5, Aesthetic Improvement 3/5, Modernising 1/5, Skill Req. 1/5
A relatively quick and painless way to add bling to your machine. Relatively simple to fit too. My machines are chromed (and a couple gold chrome). Some special editions have coloured legs/rails etc. Looks good and can refresh appearances. Look to custom powder coating locally or polishing to save cash.
New cabinet/Restoration/Decals
Cost 5/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 5/5
Getting a new cabinet can be expensive and also take a fair bit of time. You need to be serious enough about a restoration to go ahead! Cabinets are not cheap, nor are quality decals. Restoration is an opportunity if the cabinet damage is relatively light. Players just won't care.
TECHNOLOGY
NV Ram/Battery holders
Cost 1/5, Value 3/5, Aesthetic Improvement 0/5, Modernising 2/5, Skill req. 2/5
Generally a chip replacement to save worrying about battery leaks on your MPU board. A half way house is to get a cheaper battery holder which keeps the batteries and any leaks away from precious circuit boards.
REPLACEMENT BOARDS
Cost 4/5, Value 4/5, Aesthetic Improvement 0/5, Modernising 2/5, Skill req. 4/5
There's only so many repairs that old boards can take, before, much like an oft conking out car, its time to invest in something better. Generally replacement boards are well made, easy to fit and more efficient. They allow you to get going quick and fortunately fairly easy to get hold of these days.
Colour DMD
Cost 4/5, Value 3/5, Aesthetic Improvement 5/5, Modernising 4/5
This is one mod I do for all of my machines, I think they just look so much better with the dmd in colour. It modernises the game and brings more appeal. Its a fairly costly addition but does add value to your machine. It's also easy to add/remove or shift to another machine. My experience is that they are generally very reliable and I much prefer the LCD screens over the LED ones.
Support from colourdmd.rom is nothing short of superb and you can get them in the uk too, with great support as well (pinballdisplays.co.uk)
Pretty straight forward to fit unless you have to move the screen position or have a fiddly back box lamp board, but there's always a solution.
Shakers
Cost 2/5, Value 3/5, Aesthetic Improvement 0/5, Modernising 3/5, Skill Req. 3/5
I'm a big fan of shakers and have added them to my machines if they are available (or still available - note IJ). Much like the shaker in a Playstation game, force feedback to climatic events in the game adds to the experience. Sometimes it's not used to great effect and sometimes overly used, but I'd rather have it than not. There are some side effects when alls not well, with vibrating glass, things shaking loose etc. Can be a tricky install for a novice.
Pinsound
Cost 4/5, Value 4/5, Aesthetic Improvement 5/5, Modernising 5/5, Skill Req. 2/5
When working correctly with a well made mix, Pinsound adds an amazing modern digital sound to your game. I've got 4-5 of them and the biggest improvement is with Pre DCS wiliams or Data East. WMS WPC 95 games and DCS games you'll have great audio already. For me there's really a lack of good enough mixes for all machines, simply due to copyright and the time it takes to record so many sounds and re-orchestrate a game. But when it's good, it's really really good. Also the controls on the pcb allow tonal effects on the sound and the backbox/cab balance. Headphones are also possible. It's generally a doddle to fit and get working.
I'd say that a collector is after perfectly functional, mint or near mint tables. Selected improvements that make the table shiny but are easily removed/replaced without permanent modification necessary a bonus.
A tinkerer is an enthusiast who enjoys improving and modifying machines.
A player is someone who just lives to play, they care that the machine plays well and smoothly and if they are interested in a mod, it's something that affects gameplay and all aspects of it first and foremost.
I must add that I'm a collector/tinkerer/player in probably equal parts. I love to fiddle, tinker and improve things, learning a ton along the way, when I'm happy with a machine then I tend to put the time in playing it, but largely not before
I enjoy seeing others enjoy the tables too.I take a lot of pride, probably too much (!) in making my machines look, sound and feel as good as they can be. I really try and make things shine in every respect so here's a guide to modding and improving your machine in my experience - likely more useful for people new to the religion.
Anyway, here's various mods and my opinions, in no particular order. Apologies if you don't agree or if I've missed a mod/upgrade, thats what the comments are for.
PLAYFIELD
Invisiglass/PDI glass
Cost 2/5, Value 2/5, Aesthetic Improvement 3/5, Modernising 0/5, Skill Req. 0/5
An easy improvement and looks lots better, pity its generally quite expensive. I install it on all my machines though. Not always easy to find in the UK.
LEDs
Cost 1/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 3/5, Skill Req. 2/5
I take bulbout of all my machines and replace them with good leds (comet/cointaker 2/4 smds). I don't care so much about the back-box. I think that doing them yourself rather than ordering kits allows you to experiment with how a table can look and this can be great or atrocious. The advantage of less power and long life live alongside ghosting, flickering - although this is much less reduced by used Led OCD. There are many great and exhaustive led guides elsewhere
Playfield Spotlights
Cost 1/5, Value 4/5, Aesthetic Improvement 3/5, Modernising 1/5, Skill Req. 3/5
Cheap and great for spotlighting dark areas of your table. You can buy ready made mods or build them yourself. Depends on your location lighting.
Wire Lights
Cost 1/5, Value 2/5, Aesthetic Improvement 2/5, Modernising 1/5, Skill Req. 3/5
In my experience I've found these fiddly and need for the table to be in a dark room. Not for me and I just don't think it adds that much, but would work on something futuristic like Tron.
Mylar covering
Cost 1/5, Value 3/5, Aesthetic Improvement 1/5, Modernising 0/5, Skill Req. 1/5
You can use this covering in patches (large or small) to counter table wear. I've not felt the need to add mylar to my own tables.
Convulux/Plastic Protectors
Cost 1/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 2/5, Skill Req. 2/5
A recent addition to my tables that add aesthetic value and protection to common plastics, which can be expensive and/or difficult to source if they need replacing. I really like the yellow, red and green neon - but I was disappointed with the blue ones. I've found also it's better around superbright wide leds. I love them on my AC/DC (yellow) and AFM (yellow). Red looks good on TWD and MMR. I'd probably avoid the sets that just comprise of 3-4 pieces compared to 7 and up on some Sterns. Big sets are quite pricey and you need to be comfortable with removing screws, ramps and stuff to fit them.
Cliffys/Protectors
Cost 2/5, Value 3/5, Aesthetic Improvement 1/5, Modernising 1/5, Skill Req. 3/5
Largely speaking look at protecting ramps and common plastics that take a battering. Also drop holes and chutes. A plastic is generally easier to replace than the playfield. Can be fiddly and again you need confidence removing playfield parts in some cases.
MODS & TOYS
Cost 2/5, Value 3/5, Aesthetic Improvement 1/5, Modernising 1/5, Skill Req. 3/5
I've got a lot on my machine and its worth using the web for recommended ones. Avoid cheap tat (although I like the UV flashing backbox strips that are triggered by flashers) and the right mod with lighting/animation can add a great deal. If you're unsure if a mod will look good, then ask 2-3 other players, there's a lot of very poor mods out there. I know I bought plenty of them when I was somewhat more naive. They can also cost top dollar, so shop around. I went mad with TWD LE, because there's some great mods for it.
Replacement playfield
Cost 5/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 5/5
Difficult to get the one you're after and there's a fair price to pay, not only for the board for the restoration thats likely to follow.
Mirror/Decal blades
Cost 2/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 2/5
A fairly easy add to most playfields and can mean an instant improvement as mirror blades reflect lights and flashers, or themed insert blades make the game look very cohesive. If you have them fitted, be wary lifting the playfield and scratching them. You will swear. A lot.
Aprons
Cost 4/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 2/5
Options for new overlays are good, even if not many tables are covered. I've got a couple of more expensive aprons (metal) and even illuminated ones. Look good but likely that most will just go for nicely printed game cards for their original apron.
BACK BOX/CABINET
LED OCD
Cost 2/5, Value 2/5, Aesthetic Improvement 3/5, Modernising 1/5, Skill Req. 3/5
It's funny that you don't notice the effect this card has until you take it off. A pretty remarkable PCB that smooths your LED inserts and giving them back the glowing/pulsing qualities of original glass bulbs. One of those once you have it, you need it in all. However it's pretty expensive for what it is and players are unlikely to notice, hence the OCD name I guess. Note that only inserts are affected, to do GI (general illumination) the board below is required in addition. You need to connect to a pc to make the most of it and the software is quite flakey to get to run smoothly.
LED OCD (GI)
Speaker Upgrades
Cost 3/5, Value 2/5, Aesthetic Improvement 1/5, Modernising 1/5, Skill Req. 2/5
Popular upgrades that aren't cheap and to be honest the "upgrade" isn't all that, save for generally controlling the balance mix between back box and cabinet. I've upgraded a few machines but always been somewhat disappointed with the actual improvement.
Speaker Lights
Cost 1/5, Value 3/5, Aesthetic Improvement 3/5, Modernising 2/5, Skill Req. 2/5
I've got a couple to fit, but the fact they were at the back end of my purchasing says a lot about them. It adds visually I guess, but money is better spent elsewhere.
Under Cabinet lighting
Cost 1/5, Value 3/5, Aesthetic Improvement 2/5, Modernising 2/5, Skill Req, 2/5
Relatively cheap addition but I can't fathom how it would look in machines close together and using different colours. Not for me.
Toppers
Cost 3/5, Value 3/5, Aesthetic Improvement 2/5, Modernising 2/5, Skill Req, 2/5
Can add value and interest to a machine, but I think are expensive. Some machines need the original topper to maintain value.
Custom rails/armour
Cost 2/5, Value 3/5, Aesthetic Improvement 3/5, Modernising 1/5, Skill Req. 1/5
A relatively quick and painless way to add bling to your machine. Relatively simple to fit too. My machines are chromed (and a couple gold chrome). Some special editions have coloured legs/rails etc. Looks good and can refresh appearances. Look to custom powder coating locally or polishing to save cash.
New cabinet/Restoration/Decals
Cost 5/5, Value 3/5, Aesthetic Improvement 4/5, Modernising 1/5, Skill Req. 5/5
Getting a new cabinet can be expensive and also take a fair bit of time. You need to be serious enough about a restoration to go ahead! Cabinets are not cheap, nor are quality decals. Restoration is an opportunity if the cabinet damage is relatively light. Players just won't care.
TECHNOLOGY
NV Ram/Battery holders
Cost 1/5, Value 3/5, Aesthetic Improvement 0/5, Modernising 2/5, Skill req. 2/5
Generally a chip replacement to save worrying about battery leaks on your MPU board. A half way house is to get a cheaper battery holder which keeps the batteries and any leaks away from precious circuit boards.
REPLACEMENT BOARDS
Cost 4/5, Value 4/5, Aesthetic Improvement 0/5, Modernising 2/5, Skill req. 4/5
There's only so many repairs that old boards can take, before, much like an oft conking out car, its time to invest in something better. Generally replacement boards are well made, easy to fit and more efficient. They allow you to get going quick and fortunately fairly easy to get hold of these days.
Colour DMD
Cost 4/5, Value 3/5, Aesthetic Improvement 5/5, Modernising 4/5
This is one mod I do for all of my machines, I think they just look so much better with the dmd in colour. It modernises the game and brings more appeal. Its a fairly costly addition but does add value to your machine. It's also easy to add/remove or shift to another machine. My experience is that they are generally very reliable and I much prefer the LCD screens over the LED ones.
Support from colourdmd.rom is nothing short of superb and you can get them in the uk too, with great support as well (pinballdisplays.co.uk)
Pretty straight forward to fit unless you have to move the screen position or have a fiddly back box lamp board, but there's always a solution.
Shakers
Cost 2/5, Value 3/5, Aesthetic Improvement 0/5, Modernising 3/5, Skill Req. 3/5
I'm a big fan of shakers and have added them to my machines if they are available (or still available - note IJ). Much like the shaker in a Playstation game, force feedback to climatic events in the game adds to the experience. Sometimes it's not used to great effect and sometimes overly used, but I'd rather have it than not. There are some side effects when alls not well, with vibrating glass, things shaking loose etc. Can be a tricky install for a novice.
Pinsound
Cost 4/5, Value 4/5, Aesthetic Improvement 5/5, Modernising 5/5, Skill Req. 2/5
When working correctly with a well made mix, Pinsound adds an amazing modern digital sound to your game. I've got 4-5 of them and the biggest improvement is with Pre DCS wiliams or Data East. WMS WPC 95 games and DCS games you'll have great audio already. For me there's really a lack of good enough mixes for all machines, simply due to copyright and the time it takes to record so many sounds and re-orchestrate a game. But when it's good, it's really really good. Also the controls on the pcb allow tonal effects on the sound and the backbox/cab balance. Headphones are also possible. It's generally a doddle to fit and get working.
