I'd have to say I'm still on the fence - and I've been using Vista Ultimate x64 for 3+ months now.
Aesthetically, the stock Aero interface is sleek and very pleasing. However on XP using various UI utilities from 3rd parties I had an equally pleasing UI at substantially lower system resource usage.
As for performance, there is no question that Vista is slower in nearly every game when compared to XP (at least from my personal experience), albeit if it's only a few frames.
To be quite blunt, I had to do a fair share of tweaking to get the performance of Vista to be snappy enough for my tastes, disabling indexing and various other tweaks readily found on the internet.
One thing that I cannot complain
too much about is driver support. I have as of yet not had any driver issues with my system -
when they're signed. When you don't have a special approved driver Vista refuses to use it, while they may have viewed it as a great security feature for most people - it's frankly a pain in the ***. I can't use my old copy of ZoneAlarm though

NOD32 runs great however
However I'm one of those people who always wants to have the new shiny toy. And for those considering upgrading to Vista I offer this viewpoint:
Vista is like a perpetual beta of sorts.If you are looking for sheer performance - don't upgrade. If you are not fairly confident in your computer skills - don't upgrade (or at least wait for SP1). For the rest - Vista is a resource hog with a pretty UI, give it time and it may gain a good reputation on par with XP but only time will tell.