3.5 was the first really playable official D&D
I started out with OD&D in 1975, but pretty quickly got involved with the Warlock D&D rules developed at CalTech and MIT and distributed on the ArpaNet. Compared to OD&D and the various homebrews that dominated the late 70s, Warlock was very flexible, and way ahead of its time. For example, just as OD&D spellcasters gained spells, Thieves gained theivish abilities. This allowed thieves to specialize: some might emphasize traps at the expense of combat, or different combat styles, and so on. Our group extended this concept to the other character classes, and so individual characters were very distinct. This back in the 70's!
I was very unimpressed by AD&D, first or second edition, and Warlock proved a stable enough platform to campaign in for well over a decade, so that's what our group played. Just about when 3.5 was being introduced, I acquired a professional interest in playing (I was working in a game store again), so I started 3.5 and was very impressed. In my opinion, 3.5 was the first truly rational playable official published version of D&D.
Between skill points and feats I found characters to be as customizeable as our homebrewed Warlock had been, but with a much larger player base and product support. I played 3.5 continuously since its introduction, and still do. While I am very aware of its numerous flaws, it still plays better in practice than any other version of D&D I have ever played.
While I am impressed with what 4E attempts to do, and continue to play it (I now work for a different game store), 4E has some flawed assumptions, and is not consistant with its own stated goals. Combats are too long, and characters are too similar to one another. While technically offering fewer options in combat than 4E, I find 3.5 combats to be shorter, easier both to play and DM, and 3.5 characters more interesting outside combat.
I find the contrast between the two popularity curves on the 3e and 4E polls rather suggestive. While the 3e results are massed towards the LOVE IT end of the scale, with LOVE tapering slowly off to the HATE, the 4E results are crowded to either extreme, with almost no one in the middle. Either you hate it, or you love it.
EDIT: Poll results are VERY different now than when I started composing this post. Looks...fishy.
For me, I've reached the point where a new 4E book simply isn't worth carrying in my backpack to the game, or my suitcase to the con, whereas I have all the 3.5 material I could possibly want, including and especially the exceptionally fine Ptolus: City by the Spire, by Monte Cook. Compared to 3e, the support materials so far for 4E are pitiable, both in quality and quantity. While there certainly was a lot of OGL/D20 dreck, there are some fabulous gems, and nothing comparable for 4E: not an adventure, not a campaign, not a single source book that makes me go, Wow. Even the D20 stuff produced after 4E is better than the stuff currently being produced for 4E. Too bad, too, because there are lot of worthwhile ideas embodied in the design of 4E.
Smeelbo