First, let me say I'm not a power user when it comes to Office. I started with Word 6 (sheer heaven after Works x.x) and have chewed my way through versions 97, 2K Pro, XP Pro, and now 2003 Pro. mostly I use Word, Publisher, and Access. Office 2003 was a nice improvement over Office XP. I liked the UI and the way it handled.
I've been beta testing Office 2007 for a while. It's a BIG change. The UI is very different, and a little tricky to get used to. A lot of familiar menus are gone, replaced by the infamous "ribbon". The UI's very pretty, easy on the eyes. It adds tons of code, of course. After a while I got used to working with everything, although I'd get lost every so often trying find out how to do something familiar.
For power users, a lot of things about the program may not be a problem. What the new UI has done is bring some of the buried aspects of Office up to the surface. Casual or new users of MS Office may love it; between the eye candy and the sort of pictorial aspect of the programs, it might be fairly easy to learn fresh.
The experience was fine, but I'm staying with Office 2003 for a while. It does what I want, when I need to do it.