Be warned that operating systems are a very "personal experience" thing. What you like and what your experience ends up being depends on you, your machine, your apps, and a bunch of other factors. So the opinions of others are not very likely to be useful.
But since you asked, and within that caveat, here's mine:
I've used Vista. Extensively. IMNSHO, it's a pain in the rear. So much so that I removed Vista Ultimate 64 from my main box (AMD64 x2 3GHz, 4GB RAM, 2TB disk, dual GTS 7800) and reinstalled XP Pro 64. I'll eventually reinstall Vista, but I'll wait another 10-12 months before I try it again.
That being said, if you're not highly technical on computers, don't make many changes to your software or hardware, and will be keeping this machine for more than 2 years, I'd suggest you go with Vista. The PITA factor will diminish over time as they fix the most egregious errors and reduce the crap code somewhat. And XP will slowly become harder and harder for you to support. Not now, but in 2-3 more years.
If you're going to use this hardware for 2 years or so, maybe less but not much more, then I'd suggest you go with XP instead.
I'll probably go with XP64 until (as babyjax14 said) Win 7 comes out. I'm very familiar with xp and am not a fan of the vista interface constantly trying to protect me... from myself I suppose if I have to get used to a new OS gui I'll wait until either Win7 is out or disable everything I can find in Vista.
I suppose if I have to get used to a new OS gui I'll wait until either Win7 is out or disable everything I can find in Vista.