3.0 still gets my Vote!
Have been running a 3.0 campaign since the summer of 2002, and other than a 6 month break while I was at Fort Leonard Wood on active duty in the Army, it is still going strong. Someone else DMed a short campaign while I was gone, and they had begun incorporating 3.5 rules into that. My first session back in my campaign(this February) was kind of a mess, as I allowed a couple of players to use their 3.5 books (which I hadnt purchased while I was gone). My fault, as I assumed that the things that were not broken before (i.e, ERRATA) did not need changing for 3.5. Boy was I wrong, and that session became a horrible nightmare of slow combats and rules consultations that irritated me to no end. After that session I took a straw pool of everyone in the group and only one was even mildly in favor of switching wholesale to 3.5.
All in all for my on going campaign, in which the characters are average 13th level, I felt it was too much change and would disrupt the game quite a bit.
So we now play solely 3.0 in my campaign, which is nearly the sole campaign for our gaming group. We play a one-shot adventure or side trek once in awhile if a couple of people cant make it to a session.
And so far only one person has even gotten rid of their 3.0 books. Only half the group has purchased the 3.5 core books, and the rest seem to be in no hurry to do so. Of the ones who have purchased 3.5, there doesnt seem to be a rush to switch to it.
Nevertheless, I am sure that sooner or later, but not until my campaign has run its course, we will switch to some fascimile of 3.5, probably with many house rules from 3.0 that we favor.