I installed the new version of Microsoft Office for Mac on Saturday and spent a bit of time with it on my flight to Boston on Sunday. (Check out the official CNET review here.) While I like the cleaner interface and some of the new fonts, overall I feel like the applications are much slower. I also found out the hard way that the default save state of .docx screws you pretty much immediately if you aren't careful. (To Microsoft's credit, you can easily change this in the preferences.)
I mainly used Word as I was afraid that Excel would jack me up. One positive for Word is that creating themes for documents has become much easier. One negative is that my documents would occasionally disappear between Expose windows and show up in the "window" menu but not on screen. Having spent a fair amount of time on a doc, I was pretty shocked when it all of the sudden disappeared.
The big thing to note is that the Mac version of Word and Powerpoint are significantly more user-friendly than the new Office for Windows. And while we all expect Mac software to be easier to use, it doesn't make a lot of sense that Microsoft wouldn't have applied the cleaner interface to the Windows product.