I got started playing 3rd edition around 93-94, the saga lasted around 2 years. As a player, my exposure to the rules was actually relatively limited: our GM wrote a summary booklet and gave us copies. I hadn't really noticed anything was broken.
Later, I bought myself the 4th and now the 5th edition, though I have only had the opportunity to play in a short-lived 4th edition saga. I haven't been able to play 5th yet, though I do like it significantly better.
I have only played fourth edition by now, and I liked it.
But I have read the core book and three supplements (GotF, HoH:TL, RoP:tD) of the fifth and the comments on the rest, and up to now this seems like a real improvement of a good product. As I don't have a Ars troupe at this moment those parts which are still missing are no problem for me, but they would be if I were to convert our old saga, and I might miss them even in a new one. From this point of view I would be glad if Atlas managed to speed up the production of new books, but my financial situation wouldn't be improved by this.
About older versions: I never had a look into the second one, but read a (probably illegal) pdf version of the first - just a bit too plain for my taste.
And I don't know the third edition's core book but own some supplements (Rome, Maleficium, Pax Dei) and read some more: The illustrations are horrible, and if the other tribunal books are like Rome there are by far too many demons. I think there they introduced the idea that the 'holy guys' were right in the end (wich unfortunately is still afloat in the current edition), and the only real counterweight seems to have been reason (rather uninteresting before at least 15th century settings).
So I clearly prefer the fifth edition with the little change that heaven is not automatically superior to the rest and all realms compete for the world - each using its own form of propaganda.
I've been playing since first edition and the present eddition of the rules is the one that I like best with the exception of the combat system.
I don't see what the problem was with the combat system in second edition and third edition (which was more or less the second edition system) was. I found that it ran quickly and was a lot of fun. Fourth edition's combat was horrific and fifth edition combat is serviceable but compared to the 2nd/ 3rd ed system it takes longer, provides less excitement, and is harder on my suspension of disbelief.
Fifth edition does so many things so much better than previous editions that I doubt you'll find too many supporters here (this list is probably self selecting to a certain extent).
Here are some of the things I like better about fifth than any previous edition:
house descriptions (both in the main book and in the house books)
arcane/ sympathetic connection penetration bonuses
virtues and flaws
Item creation rules
boons and hooks for covenants (Just a better list of options)
I was introduced to Ars Magica in, I think, 2nd ed. and played it very briefly. Years and years later I started a game in 4th. There were many, many things about running that game (especially as a more or less new-SG) that aggravated me.
I love all the changes in 5th edition. I wish it had come out two and a half years earlier so I could have missed 4th entirely.
I happen to share pretty much all of Erik's reasons, too, but I get to add one of mine: treatment of the divine. I deeply appreciate the way RoP:D balances the Divine as the ultimate power with room for earthly religions (and divine characters) to be ultimately wrong. I hated Pax Dei so much for the way it made mages (the supposed stars of the game) utterly powerless in the face of every county priest or monk or crusader with an attitude, and the way such characters were able to freely abuse their powers and land terrible curses right, left, and center, as long as they heeded the Church's party line, while faeries and mages were condemned because of their nature, and the supposed loving God obviously sanctioned all of it.
I'm quite curious to see what 5th Ed. makes of the Infernal Realm, too.
I've been playing since 2nd edition. I love 5th edition for the standardized magic rules, and the revamped setting. I agree with Erik that 2nd edition combat wasn't that bad. While my troupe always agrees to play Ars 5 rules, we almost instinctively return to 2nd edition combat as that's the rules we started with. I do think 5th edition combat is faster and maybe even simpler, but on a gut level we play 2nd. So I dunno. But in general Ars 5 ROCKS!
My favorite version is 5e, but I am surprised to so no 3e support. It's in my experience better than 4e flavor-wise and in some of the mechanics even superior to 5e in some ways (like simplicity), even if it did have that WoD vibe and the absurdity of Reason.
I started playing 2nd edition in 1991 and my troupe switched to 3rd edition when it came out as it fleshed things out nicely and broadly improved what was a good set of rules. I've been playing in two long running campaigns using 3rd edition and, while my choice may be influenced by sentimentality, 3rd edition remains my favourite, though clearly lots of other people's mileage varies! That said, both 3rd ed campaigns I play in have binned Reason and have enough house rules that I suspect a "pure" 3rd edition game would come as a bit of a shock.
Anyhow, I didn't like 4th edition much, IMHO it lost touch with the whole 'magic is an art not a science' theme. 5th edition however appears to be a significant improvement and there's a lot of good stuff in it, enough for my friends and I to start a new 5th ed campaign, so maybe once I've played 5th ed for long enough I'll recant my 3rd ed OTE heresy!