A few months ago I used a demo version of the ABC Amber Text Merger
(http://www.thebeatlesforever.com/processtext/abcmerge.html).  It's a
small program that was pretty user friendly.  After you use it, you
need to run some additional steps -- primarily to get rid of the ads
for the software that get embedded into your new document -- but that's
a pretty easy find/replace issue, provided that you don't have images
in your document.

I merged just over 13,000 Word documents into about 120 merged files in
about an hour or so.