I have been using Microsoft Office 10 Beta for a few months now. I can tell you it is the best version of Office. It should be available in stores by June 2010. Microsoft is offering a Technology Guarantee:
Purchase and activate a qualifying Office 2007 product between March 5, 2010, and September 30, 2010, and you can download Office 2010 at no additional cost.
I’ve never upgraded to Office 2007. I still have the Office 2003. The good part is that when I installed the Office 2010 Beta, I did not have to uninstall Office 2003. These two versions co-exist.