I will say this much in response to those saying "it's not necessary" and the like (looking at you minty, Evak, and those who will follow this path like a bunch of spoons because they haven't actually used Vista and don't want to look like they hate it): there are a lot of nice, little things that add to the overall improvements that Vista provides. On the surface, not much has changed, whereas a lot of little stuff has changed that makes everything a lot nicer. I suppose this is what XP should have been.
Actually i have used it. I have my computer, that has xp, and then my family computer, which runs vista. And once again, ill have to say, it is not necessary.