Have been using it (MS Office 2007 Beta 2) for a few months.
At first I liked the novelty of the eye-candy. Now I dislike having the nuisance of altered menus, that slow me down...I already struggle enough with keyboard stuff.
When OpenOffice is stealing market share, it seems a marketing blunder for Microsoft to change menus which require customers to re-learn, since that kills more of MS' advantage.
The new docx format is xml, and makes a smaller file size than the doc format.
I just now did an experiment, & opened a 1-page .doc file, size 33 kb.
When I saved it as a .docx, it's 20 kb, or 39% smaller.
An Excel file is 1,399 kb in .xls, but is 798 kb in xlsx, or 43% smaller.