man, some old useful software can't open in vista. So maybe you have to buy the latest version of the software and it will cost more money.
Can't speak for myself, but all the people I know that bought pc's with Vista installed changed to Vista.
XP is faster even on new hardware, but If you buy a new machine with quad core processor then you don't have a choice and have to go for vista. Personally I prefer XP but been force to run vista because of processor and the need for MCE which looks better under vista. But I am far far away from being pleased about it. My machine is so slow and quite annoying.