I downloaded it and installed it yesterday (it does make me feel special, sadly, running the beta...
) and did a custom install as I did not want to risk using the Outlook 2007 beta. Running the custom install, you can opt to keep your current installation and install the beta to a different directory. This allows all the components of the beta (EXCEPT OUTLOOK) to co-exist with earlier Office installations (97, 2000, XP, etc.) so you can play around with the beta without jiggering your own Office installation. Caveat (at least the way I've done this under XP Pro Sp-2): if you try to run an older version of an app that was installed as a beta, the installer runs before the app opens. That's a bit annoying AND worrisome.
So far, so good, but pay attention to Woody Leonard's note above about the beta being slow in some instances. Example: opening a Powerpoint presentation and selecting start slideshow results in at least a 20 second lag on my Centrino notebook with 1024MB RAM and 64MB shared video memory.
Finally, I think that another reason that MS is releasing this public beta is to get people hooked on Office 2007's interface. It's pretty slick, and I can see that after using it for two or three months it might be difficult to go back to an earlier version. I like the new ribbon feature and the adaptive menus. I DON'T like the fact that only Word, Excel, and Powerpoint get it, though. It really affects the continuity of the user experience when switching between, say, Excel and Access. I find it a bit jarring and it interrupts my workflow.
LAST POINT: Heed my warning above about Outlook! All components of the beta will happily coexist with earlier versions of Office EXCEPT Outlook. Trying to install Outlook 2007 using the custom install described above will result in your earlier version being uninstalled first!!!