We are in the process of designing a new User Interface for our main application. Opinion is divided in our office as to whether we should use a ribbon bar interface or not. I personally prefer it from the point of view that it looks very modern and clean (we only have 20% of the buttons you would find in excel 2010 so is a lot less cluttered), but my boss says that end users like 'Gladys in payroll' will be confused by it and he prefers an updated version of the menu bar we are using.
I think that as more and more people move to office 2007+ then the transition will be easier, but then is MS decide to change to something else it may start to look confusing / dated again.
One of the road blocks to having a ribbon is the fact that it can introduce additional mouse clicks: In Excel 2007 to sort data a->z you have to click on the Data tab then on Sort, in Excel 2003 the sort button was always visible (providing you had enough screen real estate) so was only one click away.
Ribbon bar saves me from describing menu buttons like Menu > Edit > Image > Resize and simplifies it to Ribbon > Image Resize (where edit ribbon bar is default visible bar) so it actually help a lot with usability. On the other hand, it adds a quite an overhead with all features (~10MB using 3rd party component). I've recently upgraded one of our control system interfaces with ribbon bar, though didn't fill in the bar yet: http://epics.codeplex.com/ General reception is quite positive.