At my current position, I got a new laptop which happened to have Office 2007 on it, so I've been using it. The first big annoyance was the fact that it saves to the new xml based format by default, making inter-interoperability nigh-impossible.
Anyway, back to the ribbon - I remember after Tony first posted his blog entry, talking with him about it, and commenting that for power users, the ribbon would be detrimental, making it harder to do things. I've tried to give the ribbon a fair shot, but a month in with light to medium usage of Word and Excel, I've got to say I'm not a fan.
The first major issue is keyboard shortcuts. In previous versions, I had common short cuts memorized. In 2007, they've redone the shortcuts, and when you start using an older shortcut, you get a message that says "Continue typing the 2003 Office sequence or press escape to cancel." So, I finish out my sequence, and the changes happen. The ribbon doesn't change to the appropriate version for the change made. I can see why this may not happen, but it's slightly disorienting, and makes me wonder if my changes took hold, particularly if it's not a visual change.
On the plus side, I can hold the alt-key down and after a short delay, get a little, hovering, information things showing the shortcuts for whatever is on the ribbon. While this is fine if I have the time/energy to relearn things, it's not a great solution.
I also find myself poking around each of the ribbon tabs, trying to find things, and find the hierarchy confusing at times, particularly for those items that have been re-classified.
Overall, the ribbon seems to be a big drawback for anyone who's accustomed to using keyboard shortcuts as opposed to using the mouse. While the icons and big buttons might be great for mouse users, they don't provide any advantage to power users who use the keyboard, and with the remapping of previous shortcuts, proves detrimental.