I am using couchdb in a few scenarios, as a document store for http://devk.it (under development) and in a much larger scale as a template store for a distributed email delivery system.
CouchDB is very slick for what it does, but I was not able to get it to run at as high of a concurrency level as I would have expected. Also note that the maximum document size is fairly limiting at 1MB due to the hardcoded max input buffer size in mochiweb. You can however alter a header file and recompile to get around this limit.
