Ok well I've got mine al up and running now. I'm running it on my HTPC box, so I'm not really testing anything at all except the MCE aspect.
After it installed, everything is working swimmingly. It didn't auto-install my three total tuners, but a simple right click and update drivers in hardware manager seemed to install everything. Same problem/solution with onboard realtek ac97 audio.
My old (MCE 2k5) box did well with high def, but there was a little bit of lag at times. I was previously using an Athon64 3200+, Geforce 6200, 512mb ram. Now with the upgrade to vista, it didn't like those specs quite as much. I've upgraded to a 6800 ultra and 1gig and am happy to say performance in HD far surpasses 2k5 with my previous specs.
Unfortunately my Soundblaster Audigy 2 didn't like to be installed, but I havn't tried downloading drivers from the site. Right now I am stuck with my onboard Ac97 which means no surround sound. The FM tuner installed nicely, but it sounds very bad. It might be the weather, might be the antenna, but I'll have to examine more on this. I don't imagine MCE would alter the raw data too much before sending it to my speakers, but who knows.
The hard drive is accessing constantly. I don't know what it's doing, probably indexing, but I don't like so many cycles 100% of the time on a machine that already sees overly heavy HD use (buffering sdtv or worse hdtv keeps a hard drive busy).
I was unable to connect this to my Xbox (old, not xbox360)... I've never tried to get the xbox to connect to anything before so I'm not sure if it's the xboxs problem or something stopping it from connecting in MCE. For what it's worth, the xbox is chopped up and modded to hell with huge HD, XBMC, and all the other goodies you can cram on to one of those boxes.
There are a few small bugs. The first of which, every time I try to "test your speakers" in speaker configuration it ends up erroring out and restarting the program...
When I VNC (installed nicely) to the machine, every time one of those dumb authentication windows pops up it will instantly kick me out of VNC and wont let me back untill I click with the physical mouse. This isn't bad since I'm sitting right there, but remotely would be a pain. I Wish I could turn this off, I have no passwords so it shouldnt be too hard... hint hint if anyone knows how

Now this is something that should have been fixed. NONE OF THE DEMO CONTENT WORKS! There are all these like National Geographic or Apollo 13 or Vertigo teaser clips that come with, but none of them work at all. They all flash some error message about content expired and error out MCE hardcore (out completely instead of a soft-reset like audio test does)
Sorry this is somewhat of a jumbled spaghetti post but I am just writing what i am seeing or thinking as I see or think it

Anyone have any questions on the features of new MCE feel free to ask. Heres my specs:
AMD Athlon64 3200+
2x512MB PC3200 Geil ram
Geforce 6800 Ultra in AGP
WD 250gb 7200rpm x 2
HP USB 2.0 Dual Tuner/IR/Radio (Built by Adaptec)
AVerMedia AVerTVHD MCE A180 HDTV PCI TV Tuner, big ass antenna on my roof.
Dont remember brand DVD +/- RW
(UNWORKING) Soundblaster Audigy 2