will say this much in response to those saying "it's not necessary" and the like (looking at you minty, Evak
I quite like Vista and have been one of the few supporters the past year. I have tried it but it doesn't offer much worth paying for at this point.
I just want my primary apps to run as fast as possible with as little resources being waster in the background. My primary use for an OS is to run applications, the two I use most being 3ds Max, Photoshop, Bmax, Ogre3D and flow.
My system is pretty old, CPU about 4 years old Athlon64 3000+ 754 with a recent upgrade to 2gb of PC3200 ram. Thanks to my AGP 7600GS vista's UI is snappier than XP, but that doesn't make it better when my main concern is my weak CPU and high RAM requirements to run Vista. Especially if you play games and it's your only OS.
In my case there really isn't any reason to have both, when I get a new PC that has modern components (newer than 2-4 years old) I'll probably try Vista again.