Jensen Harris of Microsoft was the leader of the Ribbon interface UI team.
"An online survey conducted by ExcelUser reports that a majority of respondents had a negative opinion of the change, with advanced users being "somewhat more negative" than intermediate users; the self-estimated reduction in productivity was an average of about 20%, and "about 35%" for people with a negative opinion"
Jensen's index to his own posts about the Office 2007 UI:
I recall that Joel Spolsky (worked on earlier Excel and VBA) noted that MS found a significant number of enhancement requests was for features Excel already had. The Ribbon develepment was intended to make these features more obvious.