I read somewhere that MS reckon that the 2007 suite is better laid out for inexperienced Office users, grouping things more sensibly, etc.
This is actually a long story, and a well-documented one, if curious, here:
http://blogs.msdn.com/jensenh/archive/2008/03/12/the-story-of-the-ribbon.aspx
To summarize. It started with Word, Microsoft actually did a lot of testing of testing on this, like hand/eye coordination testing in the labs, etc... It worked extremely well for you average user on Word. Great.
They didn't want horribly inconsistent Office offerings, so if you ship Word with that thing, all of Office is getting it, and that makes sense. Except it just doesn't work as well, as is, for Excel and Access, per their users.
Most Word users really like it, as I understand it, it just didn't translate as well in the Excel and Access environments, as is. But it's also a work in progress, as you can imagine.
Me? I don't know if I like this thing or not. It sure is foreign, but I'll learn it with time. When I first started using Excel 2007, I actually thought 'Hmmm, I kind of like this thing'. Then I built a non-trivial Application in Excel 2007 and I thought 'Shoot me'. But, again, it's a matter of not knowing where the functionality lies, I'm back to 'this might be okay', in my mind.
I suspect if I had never used Excel before, I might actually like the Ribbon, it does expose a lot of functionality that in previous versions I went looking for/researching. It's not perfect, but I'm trying to be patient with it before I throw it under the bus.