I downloaded Vista Ultimate off MSDN yesterday. I have just finished installing now and have been playing with it for the last 20 minutes or so. I couldn't get the Betas to work for me, so I am a Vista noob.
I upgraded from XP, rather than starting fresh. It installed/upgraded without a hiccup on my Dell Inspiron 9400 Lappie. I only have the Intel onboard graphics, so things could be a little better there, but I have 2Gb Ram and 2Ghz Duo Core, so no shortage of grunt to run things. It runs about as fast as XP. Somethings take a little longer to load, some take less. Internet seems a little faster, although it is getting pretty late (12am) so that is probably as much a factor.
The warnings when trying to run Sysadmin type apps are a little annoying. The sidebar gadgets are great and have downloaded a few extra's. Once either yahoo messenger or Gaim becomes available as a gadget that will be even better.
I guess the major change is the Aero interface. I must say I like it a lot. Seeing as I have fairly lowly graphics, my system is probably not doing it justice. Having said that it is still (IMHO) more of a programmers take on how good UI should be, rather than a designers idea. KDE and OSX are still better, with XGL on KDE showing genuine ingenuity. One thing I wish MS would "borrow" from X Windows is the virtual desktop system. It is the main thing missing.
Overall though I think Vista is a worthy successor to XP and I rate it so far as a 7/10. Well worth upgrading to.