I've been using the RC since it came out.
My god, this is fantastic. Peek is the best thing to happen to the Windows UI since the Start menu. Being able to open additional instances of apps by middle clicking them in the taskbar is awesome. Shake is a hell of a lot more useful than I would have ever guessed it'd be. Love it. And good lord, per-application recent document lists, with pinnable items. This is the innovation that has finally made me willing to use and love the new start menu layout (I've been using the classic design, all the way up to Vista. I was quite the diehard).
There are so many little new details that I love. Calculator now shows recent operations, so I'm not constantly second-guessing if I put in the right numbers or not. Also, I love the way that Wordpad now has a button to launch Paint to insert a picture. I'm pretty sure that that's supposed to be a little wink to us folks who remember Windows 3.1.
As far as I'm concerned, this is the best, most useful, most functional GUI ever designed thus far (linuxtards can take their compiz and shove it). At least, it sure is for me.
I'm also pleased that I've managed to turn off pretty much all the useless crap and clutter and stuff that I don't like and don't use (for example, you're able to make the taskbar pretty much old style, the way I like it, and disable all that MacOS-dock-like bullshit; I don't like the big icons style, and I don't EVER want to launch programs from the taskbar!), with the exception of not being able to make it not list Libraries in the left-side Explorer panes (libraries suck). My only real complaint is a tiny little cosmetic detail...and that is that the taskbar graphics look awfully flat and 2D compared to Vista. I really liked the deep, blue-and-orange theme in Vista. I hope that there is eventually an option to make it look a bit more like Vista Aero, but for now, I'm set.
This thing is awesome. Bravo, Microsoft.