I tried the beta about a month ago. I'm not fully sure if it was legal, I've heard about a beta testing, but I got mine via Supertorrents.org.
Anyways, I was very disappointed. While Vistas appearance was extremely cool, I could not find enough support. For an example, Microsoft itself hadn't supplied Vista drivers for my MS wireless laser mouse. I could not get my sound card to function properly, etc. Finding drivers for my Logitech joystick was also impossible. I don't know about the current situation, month ago support from 2nd party developers and even Microsoft itself was very limited. There simply were no drivers available. Most games didn't work either, encountered problems with DirectX, but that might be because I didn't get drivers for my 3D card.
Most likely I will wait at least 6 months before upgrading. (Or until Alan Wake comes out

)