I've installed a pre-lease version of vista (RC2) about 2 months ago and ran it for awhile. It's pretty sick, but yeah it does require a fast computer if you want to get everything out of it. I'm waiting a while to make the switch from XP permanently because of bugs and stuff that still need to work themselves out. Plus, there are still a lot of devices that don't have Vista drivers and applications that wont run smoothly on Vista.
It's a great operating system, but I would wait a few months before completely switching over.