I’ve been involved in the Office 2007 Beta test of for the last several months. For the most part, I’ve LOVED it. Even though we were warned over and over to avoid using the beta in a “production environment,” that is, where it really counts, I’ve been doing just that. Later this year Microsoft will be opening up Beta 2 to the public and has is still signing people up to participate in that.
If you’re interested, it's not too late to sign up to receive Beta 2 of Office 2007.