OpenOffice

In the other corner... OpenOffice has also been using XML in its Office suite, but with a different set of priorities.
Already XML OpenOffice files are zipped-up XML, with supporting binary files (for images, etc.) when necessary. Getting into them is trivial, and requires no special Save As.
Standards-based process The OpenOffice XML is going through OASIS rather than simply appearing from the OpenOffice team.
Very different markup The OpenOffice markup has been designed with an eye to many different needs, and looks far less like a direct object serialization.
No custom schemas, yet OpenOffice hasn't yet responded with a toolkit like Microsoft's, though their XML approach is open enough that it should be possible to do things like the Excel work quite easily.
More informatiom O'Reilly started to publish a book, but sadly it never reached the presses. You can read it online at http://books.evc-cit.info/.