My wife uses Vista on her new laptop and hates it. It came preinstalled, it's slow, it's non-intuitive for anyone who's used a computer before (it might be great for folks who are just now trying out these 'television typewriter' machines, but not for anyone with technical experience as far as I can tell).
Vista might be fine in another year or so, no idea, but I have no interest in installing it on my computer right now, there's no benefit I can see. I'll stick w/ XP until I feel Vista reaches a point where it's an actual move upwards and not down.
Of course, I might have to wait until the next OS. Windows Me (Millenium) is a good example of an MS OS that just straight up sucked, it never got better.