I've been using Office 2007 for some time now, and absolutely love it!  Many people, including me, who have had a lot of experience with the toolbars, menus, context menus and general quirkiness of Office 2003 found it challenging at first to find our way around Office 2007.  Interestingly enough, my kids - who don't have a lot of experience with Word Processing programs, found their way around really quickly and were better at it than me for a while!  The biggest issue I hear about these days is not the new Ribbon UI (which doesn't take very long to get used to and fall in love with), it is the number of people still running Office 2003 and the issues their systems have in reading and writing the new Office 2007 format.

Fortunately for them, Microsoft released an Office 2007 Compatibility Pack a while back that will allow users of Office 2003 to read and write Office 2007 file formats.  I highly recommend the new format as it's basically a compressed file with XML inside that (as it's a compressed file) will take up less hard-drive space than the traditional file format.  Some folks I have talked to have seen a dramatic decrease in disk space usage by converting their files from Office 2003 to Office 2007, so you might want to run some of your own tests as well.