PC Magazine posted a nice Vista overview.
So far I have been using the media center on an Amd 2500xp with 756 (or so) RAM and wasnt very impressed. Sure it looks nice, but not THAT impressive. Also tried readyboost, and though the usb stick's light was constantly flashing it didnt make a bit of difference. Furthermore the harddisk always seems to be busy. And this was just while browsing the web with FireFox (cant stand IE7's interface or lack thereof). So on that particular PC i am back with MCE 2005, which works like a charm :).
And on my main pc, that i use to game / email / surf etc, I am not going to try it for a long time. It's pretty stable with XP sp2 and I havent seen any features on Vista that I care for...