I more or less had to use Word while creating a draft of my new Ruby book because I was required have separate chapters and a master document (and Word master documents are evil :-)
I have finished the book to the point where I have a complete manuscript and delivery-wise I can go back to one large manuscript file. My publisher's styles are very portable between Word and OpenOffice.org, so I coalesced everything into one document file and switched back to OO.o. I really had a lot of hassles using Word's master document "feature", and it feels great to have that behind me, and back to using OO.o because when I use OO.o to write, I don't even think about the mechanics of using the program, just on what I am working on. I still have 4 or 5 weeks of work on my book, polishing it based on some excellent feedback from the owner of my publishing company and the tech reviewers, but I am happier working with a single OO.o document.
When Microsoft un-cages their next version of Word, I will probably have to buy it for compatibility reasons, but hopefully I won't have to use it often.
I tend to try out many different technologies for just about everything that I work on. For writing, I was pretty much set on using LaTex a few years ago, but many publishers don't like to deal with LaTex. Same thing with DocBook.
Another thing that I really like about OO.o is that the file formats are so transparent and easy to process (if required). Proprietary file formats are stupid, or rather people who rely on the are taking a real risk. A note to people who have to use Word in their enterprise: set up backup scripts using a tool like antiword (extracts the text from Word documents) or script OpenOffice.org Writer (it has a batch option to recursively walk nested directories converting Word documents to OO.O formatted copies). Your backup script can then archive the information in your Word files. Really, do you like the idea of archiving Word files, and hoping that you will be able to read them in 5 or 10 years?