The TR includes performance improvements, bug fixes, fit and finish enhancements, and according to Jensen Harris, a lead program manager on the Office team, more than 1,000 changes to the Office interface's newest graphical element, the Ribbon. "The UI [user interface] is now totally feature complete, and you will see only cosmetic differences between B2TR and the final version in most areas," Harris wrote Wednesday in a blog entry.
Microsoft won't be releasing another Office beta before final release, at least not to the general public. So if you are really keen on trying it out, this might be your last chance to do it (legally) before it goes retail. Read more at the official Office 2007 preview site.