I have used all of the major iterations of Office since 97 due to licensing agreements with the University my wife and I are affiliated with. I recently picked up Office 2007 Enterprise edition for a mere $20 (and yes, like Darwin's faculty license, it is a 100% legit). I find that it just works. I can load any of the applications in less than 5 seconds (though I am on an extremely fast PC), and I love the live previews as you scroll through options. I have found the ribbon to be rather intuitive, and I rarely have to search for more than a few minutes to find some option that I haven't used in 2007 yet. Once I find it, it makes sense to me why it is located where it is in the ribbon, so I don't have to hunt for it again.
I did run into a nasty bug the other day, however, but it may have been a combination of Office and Vista rather than just Vista. After a couple of weeks worth of daily use, I suddenly found that when attempting to double click on any files saved in older Office format (doc, ppt, etc), Office would tell me that it was not installed for the current user and that I should run setup again. I could start all of the applications from the start menu with no problem, as well as double clicking on any file that was saved in the new Office 2007 format (docx, pptx, etc). Running setup did nothing to solve the problem, nor did Office's repair or diagnostic functions. I never did solve the problem under Vista. Instead, I finally found all of the XP drivers I needed for my machine and installed XP. Office 2007 is back to running perfectly fine.