I have been installing different operating systems for years (DOS, Win 3.1, 95, 98, ME, XP, 2000, 2003, Linux, FreeBSD, BeOS, etc) and installed Vista when it first came out. It was fun until the novelty wore off after a few months. I decided to go back to XP and I had forgotten how fast my hardware actually was. XP flew compared to a base install of Vista and I can do everything I need on XP.
I have the Vista sidebar on XP as well as a modified theme to make it look similar to Vista, but will all the speed of XP.
There are modified builds of Vista out there that take a lot of the bloat out, but I haven't tried them since I am pretty happy with my XP build for now.