I was very interested to read this article. I have been working closely with WordML for about 6 months, albeit not in conjunction with Word itself.

I have certainly felt that I was on the bleeding edge of this technology sometimes, and the lack of extensive documentation and community support has been frustrating. However, I was at least relieved that the schema documentation is accurate and the process of round-tripping word documents to XML and back is reliable.

We have come up with an online document review and collaboration system called PleaseReview http://www.pleasereview.com.

We allow reviewers to view an HTML rendition of the document, so they can interactively comment and propose changes (using a pure web interface) and generally collaborate in a way which is not possible with Word's built-in commenting features. After the review has finished, the author can download the document with agreed changes already applied, and open comments shown as tracked changes.

I think this project would have been almost impossible without using WordML, because we effectively edit the document on the server to apply comments stored in a database. So I would say we have definitely used Word's XML features with a great deal of success. Maybe you would even like to consider using it in O'Reilly's review cycle :-)