XP is great, SP2 that it is. i used vista at work and a major problem with vista is the drivers -- there is a lot of hardware out there that does not support drivers, for example we had a industrial panasonic copier/printer that was maybe 9 years old and panasonic had no drivers to support vista, so my computer (the newest one) could not print to the machine.
other than that, vista is ok, they changed a bunch of stuff around simply to make it "seem" different IMO. if you want to switch, make sure to check compatibility not only with your computer hardware/software but any other machines...
Before enlightenment - chop wood, carry water. After enlightenment - chop wood, carry water. ~Zen Buddhist Proverb