We have a huge distributed project. Users, roles, meta-data in mysql, everything else in couchdb.
Very good living.
CouchDB views allow you to do the app is incredibly flexible, without spending time thinking through complex internal relations, creation of composite indexes in the database, etc.
And due to the very handy tools for the administrator(an amazing web admin) and master-master replication literally 20 minutes connected to the additional server(100+ requests per second, several gigabytes of data, 2 servers).