Once you've used Vista for a good while and actually take the time to learn all of its new features, it's kind of a pain going back to XP ... You have to add a lot of 3rd party apps to XP to get the same functionality that you get out of the box w/ Vista ...
My only gripes w/ Vista are:
1) It runs like shit on systems w/ less than 2 GB of RAM (though it runs faster than XP does w/ 3+ GB of RAM)
2) User Account Control is seriously ... seriously ... seriously annoying ... Fortunately, it's easily disabled ...
Hardware/Software compatibility issues are inherent to ANY new operating system (happened w/ XP, happened w/ OS X Leopard), so I don't really hold that against Vista.