Vista Benefits-
Plays DX10 games
Media centre has more option (but is less easy to control with the remote) + media handling is better the basic XP but not as good as third party XP utilities.
Mainstream 64bit/4gb support.
Faster search if you have forgotten where you put things.
Prefetch is useful if you regularly use 1 or 2 large programs and want to quickly access them.
Disbenefits
It is pain to turn off all the noddy mode features (networking is the worst).
The interface is an acquired taste and you cannot get it to operate as W2k 'classic' as per XP if want something which is more efficient.
The new search system slows the whole system down for long periods and leads to a lot of disk trashing; this is pointless if you save files in the right place.
Driver availability is still not as good as XP.
SP1 on the 64 bit version of Vista has solved some problems and feels faster (bot not anything like as responsive as XP)