I used Vista in several beta forms and the latest worked very well. My PC is nearly five years old (Athlon XP 1800+, 768 MB RAM, GeForce 6600GT) and it did not bog down my system. The main problem was the lack of decent drivers for my Nvidia video card and Sound Blaster sound card. The sound card drivers simply did not work and the video card drivers performed very poorly. Otherwise, I really liked Vista.
I am going to build a new PC very soon and I will be installing Vista on it. Vista will be required for DX10 games.