Put simply,XP is so much easier to navigate and looks better with WB.
I find Vista easier to navigate. And looks just as good with WB.
Yeah Snow...that tree navigation in the Start menu is so much better than flyout menus...NOT!
With Vista I rarely have to look at the list of the programs. I open the start menu and type two or three letters of the app I'm looking for and it instantly pops up. I manage to launch apps much faster that way. Looking through the XP "All Programs" menu which quickly grows and glutters just takes too much time.
My own experience is:
The user interface is somewhat changed and you'll have to adapt. However, it didn't take long until I was accustomed to the Vista way and now I really like it. I especially like the improved file navigation. Now I find it very awkward to go back to XP and I've yet been forced to boot into XP to do a task. And I got allot of random apps installed. The only thing that's failed to run has been some very old games.
I've installed Vista on my main computer, but not on my laptop and I don't intend to either.
A piece of advice if you get Vista. Don't upgrade your XP installation, it'll takes HOURS! Back up your data and do a clean installation which is done in about 20min. (Maybe 30 if you include post installation of some drivers)