Doug Mahugh and a bunch of the standards crew (both in and out of Microsoft) have been having a great discussion on document format interoperability. They (and referenced posts) are worth reading.
Michael Kiselman has been publishing some great case studies on the use of Open XML. These cases studies are hard evidence about the uptake of the format. I really love seeing people putting the document formats to good use. Folks are building innovative solutions that really wouldn’t be possible without using Open XML.
Look for these among the studies that you find here.