My initial thoughts: Finally!*
* 7 looks and feels a lot like Vista. As I was in the beta and RC cycle of Vista, I'm familiar with where Vista was at this time of its cycle.

If you have interest in running the beta, the download may still be available.
I downloaded both 32 and 64 bit versions. For the beta, I'm optioning mainly the 64 bit version on an older box:
Asus A8N-E (939)
AMD 4800+
4Gb Corsair XMS DDR2
Plextor PX-760a
BFG 7800GT
Creative X-Fi Platinum
BT: Logitech Mouse and MS Keyboard

The box I'm using is mainly my media storage / server. I'm using a 50Gb partition of a 320Gb Seagate HDD.

The installation went smoothly and after about 12hrs of playing around, I've yet to crash.

Of the hardware, the only device that I cannot recognize, or find working drivers for is the sound card. Go figure...Creative finally released its working Vista drivers nearly a year after the release.

Nvidia, as a counter, offered an immediate beta release of Forceware 7.15.11.7923 v179.23 (DirectX 10 compliant).

Networking was simple, and although 7 wanted me to create a HomeGroup and issue its own password, I was quickly able to join my already familiar lan Work Group.

Software:
IE8: Upon 1st run, IE8 asks you to take Microsoft's Accellerators, or choose your own (blog, email, search, map, etc.). I customized to my likings without hickup or incident. No crashes yet, and although some sites need additional java and active x installs, the browser seems stable. Sysopt (writing this post in IE8), Netflix, Youtube and a litany of other bookmarks all seem to work as if nothing's new.
I use Google's Chrome quite heavily, so the only thing I 'miss' is incognito mode. Ahwell. I'm going to try installing Chrome in a few minutes...

WMP12: Absolutely no issues. It found all my media, playlists, etc., and even though I have over 700Gb of music, it was adding to the library as I was playing without crash or bogdown (Vista/WMP11 was horrible at this). I've even shared my media with my PS3 and am streaming to my home theatre as I did prior to running 7.

Open Office 3: I'm going to spend most of my time testing 7 with 3rd party apps. I started with OO3. It downloaded/installed without incident, so I'm fairly certain MSO 2007 will work fine. I've created various types of documents, opened and worked with existing docs, all without glitch.

I'm taking off for a bit now, and will be running a few stress tests while away. At initial pass, as a beta, I'm much more impressed with 7 than I ever was with Vista...which is sad, but I'm very much anticipating and looking forward to taking a lot less tylenol during this testing.

I'll post more as I play with it. If you'd like a professional UAT perspective, try reading Paul Thurrott's Windows 7 Blog.