I hope this isn't too off topic but imho....
Vista takes more resources for itself than XP, as far as new features go, they're all pretty specific and can be found elsewhere, done better, and usually for free, and the only real reason to upgrade was DirectX10 being exclusive to Vista. However DX10 has yet to show off anything that cannot be done in DX9.
Whats more there are the inevitable compatibility and stability problems that come with any new OS (give those a couple of service packs to sort out).
I use Vista for day to day stuff because its pleasant enough to use when its behaving, and I thought it better to get a OEM version included in the price of my new rig than pay three hundred quid or whatever the silly prices for it are later on. However as I mentioned I also have XP installed for gaming etc so I can live with it.
Vista is bound to be needed later on as it takes over from XP, but until its sorted and actually necessary I wouldn't bother, depends on your needs I guess though (I'm picky :D).
Cheers