I'm not one of these people who mouth off about Vista, and don't want anything to do with it because "XP IS TEH BEST!!!" etc. But I've used both, and I have to say I prefer XP. Vista looks great, but it just doesn't feel great. Maybe because I've used XP for so long, but I don't think I'm going to upgrade any time soon. Of course, both have their advantages and disadvantages. I like how Vista doesn't freeze up as often, and generally looks better and I've found it's more user friendly for the PC n00b- As long as they're not trying to do anything big.
But still, I like XP. Download a couple of new skins and icon packs, and there's really no point in upgrading.