I am finding that I am compelled to install it on almost every machine available to me. It's hard to explain exactly, but the overall polish applied to it so far makes it nice to use. I have been using Vista x64 for a couple years now and I have been relatively happy with it. So far Win7 is a nice upgrade from that; so much so that I hate going back to Vista now since in so many ways Win7 feels better.
Some of My Favorite Things:
The new taskbar is awesome IMO. Being able to pin icons and move them around at will is great. The jump lists are cool and I can't wait for more apps to support that functionality.
The new Libraries feature in Explorer is a nice idea, making it quick and easy to find different types of media all over the PC. I made a new Library for "Downloads" and have found that very useful.
The new window manager is cool, being able to dock items and move things to the next monitor. Still, I find myself having to install FreeSnap to be able to snap items to corners and such. I wish FreeSnap was built in...

Overall I am surprised with stability and hardware compatibility of the beta. I haven't had a single BSOD and all of my hardware works when I install the Vista x64 drivers. This is a great sign for the eventual release - perhaps we won't be plagued with the chicken+egg driver problem this time around!
Surprisingly, I am liking IE8. There are neat features built into it. I imagine I will still use Firefox and Chrome more, but IE8 definitely feels like a substantial improvement over IE7 for those times when you have to use IE.
The networking connection manager works a lot better than in Vista. This is great on my laptop for switching various WiFi networks, my work network, etc...
What I Don't Like
I miss the old Sidebar. Now your gadgets just go to the desktop. Since I have dual 24" monitors I found it very useful to dock all my gadgets on the side of my second monitor, where they were out of the way without sacrificing too much horizontal res. With Win7 now this is harder to accomplish since the gadgets are on the desktop, meaning apps will more easily obscure them. Certainly not a dealbreaker, but a little disappointing nonetheless.
The early buzz was that performance is much better than Vista. So far on my systems (a Dell XPS M1210 Dual Core laptop, Core i7 box, and a 8 core Dual Xeon workstation) perf doesn't seem all that much different. Not necessarily a bad thing, but it's too bad that at least for me the buzz of a leaner, meaner OS doesn't hold true.
Too bad that you have to install a hotfix due to a critical MP3 bug that went out in the release. Not a big deal but don't forget to install it!
Final thoughts:
It's too bad the beta expires August 1 as I will miss using it. This may be the first version of Windows in a long time that I will install as soon as it comes out. In the past I have waited for SP1 releases. However, if this beta is a sign of things to come I am pretty excited.
Curious to hear what others think of the beta.