Read this fact sheet, and this technical case against ooxml, then read this guy, affiliated with microsoft, and this piece on groklaw.
Then, if you've still not had enough, read about Sweden.
What a shambles.
As someone who implements OpenOffice in our document generation and workflow of a largish application, it's been a joy to work with ODF. It's simple enough to understand, and the spec is clearly laid out. I haven't read the whole thing, but I don't need to: I can easily find the information I want with only a little bit of googling.
As someone who is fighting with half of his office using Office 2007, and sending incompatible documents all over the place, I now believe that September 2 is doomsday.
How exactly am I meant to deal with 6000 pages of information hiding possibly contradictory information, if I want to implement something based on ooxml?
If Microsoft themselves can't even implement it in Office 2007 (I believe its a subset of the spec they use), what chance do I have?
Worse; I can't believe the shills who call this "Open XML" - it's Office Open XML, and deliberately named in such a way to create confusion.