... is Jensen Harris'. He's been blogging steadily on the changes in the UI in Office 12. I've been running the Beta for a few weeks now and I'm sold (mostly). I'm primarily a Word and Outlook user, and the new UI really works well in both those apps (admittedly, Outlook itself doesn't have the new ribbon, but the "write new mail" window uses a stripped down version of the Word UI). I've also used Excel a little, and some of the new table features are great (the Excel team blog has a ton of information on these new features).
Check them out.