After playing with Office 2007 beta on the Q1 for several days, I finally reverted back to Office 2003. Why? The 2007 beta was somewhat sluggish on the Q1. I suspect it's because the code hasn't been optimized for speed yet. Also, I want to play with Tablet Enhancements for Outlook and the new TEO 3 beta 2 crashed Outlook 2007. Finally, all this new stuff is crushing my productivity until I figure out how best to use all the tablet features, so I'm going for stability for a little while.
In case you care, Office 2003 is quite zippy on the Q1 compared with 2007. Sometime I'll post my thoughts on the ribbon bar and other UI changes in Office 2007.