I'm currently playing in a 3.0 with some 3.5 changes as well. I'm beginning to find this to be the way to go; i.e., combine the editions for best effect. Obviously it's expensive, but (as my girlfriend knows all to well) I'm always willing to dump my life savings down on some game stuff.
3.5 changes we've added to the 3.0 game include:
--3.5 Monster Manual, and thus DR changes...I like these because, as MoogleEmpMog said >> The wonderful new DR rules have to be my favorite. Golf bag of weapons? More like "attacks are actually effective against creatures with DR, just less so." <<
--3.5 Ranger, Bard and Sorcerer...maybe they're not perfect either, but they fit with the setting we use, so that's more important than perfect rules-balance or anything
--3.5 Psionics...I just could not get down with 3.0 Psionics. I find the whole Psionic Focus thing to be odd (outside of combat, you can theoretically just keep rolling that Concentration roll to gain it, so why not make it automatic, or more difficult?), but overall, it plays 100 times smoother.
--DEFINITELY NO WEAPONS SIZES. Why did 3.5 have to change that? It worked just fine in 3.0. Oh, and some of the racial changes didn't make sense either (Half-elf changed for what reason? Half-orc is nerfed why? Gnomes are Bards, eh?)
I like a lot of the revisions of feats and maneuvers, including the changes to Improved Sunder, Improved Trip, Improved Disarm, etc, but we don't use them at the moment because not everyone owns either the 3.5 PHB or has access to the internet (damn neanderthals! who doesn't have cable nowadays?)
3.5 has some great ideas, and although I guess I'm a stupid poseur fanboy because I bought the books, I get mileage from them, but I also think an extra 2-3 years and a solid 4th edition would have been fine. Editions changing in less than 4-5 years seems odd...releasing the PHB with errata and the occasional restructured rule as a "revised printing" wouldn't have been a bad idea, rather than redo the books for the amount of changes they made.
All just IMHO, of course