Nope, wouldn't go back:
There was a fascinating recent post on one of the many forums / Blogs I follow. The author posted a statement "from a major corporation" about how they would not be adopting vista service pack one as their OS of choice. It was found to be too slow on their hardware, have incompatability problems with other software and generally unstable.
There followed a tirade of posts about how XP was the best thing since sliced bread, knocked vista on many aspects, blah blah blah.
A day later the author then posted the actual statement. It had originally be made several years earlier about XP service pack 2 - he'd simply substituted Vista SP1 for XP SP2 in the original post to prove his point. Its amazing how quickly people forget what their OS was like.
Don't get me wrong, I have my gripes with Vista, but I had gripes with XP, 2k, NT4, 98, 95 and 3.1. Moving forward is the only way to improve.