Most of our games are 3.5, however we are still running a 3.0 game (doing the adventure path series [forge of fury, speaker in dreams,etc]) mainly because all of character made certain choices which when converted would seroiusly weaken our characters without a rebuild (we had a dwarf fighter/rogue with improved critical: scimitar and a keen scimitar, my character is a half-orc cleric who actually had some points left over if you use the standard 25 point stat buy, so I am dependant on the 1/hour per level buffs), however the GM has adjusted a couple of the spells (heal, harm and blade barrier) to 3.5 and used clarifications from 3.5 rules on the various Bigby's hand spells (which I get as domain spells) ie them being force effects.
Overall I really like 3.5, when I heard about some of the changes I was very dubious about them (particularly the DR vs Materials) however when I actually played them, the game is much better. Demons and Devils are not now the pushover/XP cow they used to be in 3.0. The only thing I'm not keen on is the rule for incorrectly balanced weapons based on size, I don't think the rule is wrong, but it does mean that halfling/gnomes get a bit stuffed, when it comes to handing out magic items (Hmmm...next time I play a small character magic user, I have to invent a spell which permanently changes the size/balance of weapons).