Office 2007 was the first implementation of the "Fluent" UI, but it is not necessarily the best. This is the doc that convinced me to use the ribbon for a UI redesign:
The goals of the Fluent UI design matched our needs almost perfectly. I think it works better for us than it does for Office. Many things seem rushed or forced in the Office 2007 implementation of the ribbon. Maybe Office 2010 will better deliver.
(It is interesting to note that this doc talks about the ribbon being custiomizable by the user, which is decidedly NOT the case in Office.)