I am already excited about document databases and especially about CouchDB\'s simplicity. But I have a hard time understanding if such databases are a viable option for multi us
Multi-user systems do not require relational databases, though RDBMSs are a staple technology for data storage/retrieval for a vast number of (especially CRUD) applications.
If you want to read-up on document/object -oriented, distributed database solutions of yore, search on "Lotus Notes/Domino" (it's a mature technology/product in this area that's good background knowledge in how applications are designed in a document-based paradigm. Classically, it's really good at workflow type applications).
On CouchDB specifically, check out:
http://wiki.apache.org/couchdb/ (this shouldn't be a surprise)
http://seanoc.wordpress.com/2007/10/12/more-on-couchdb/ (easy reading description overview)
http://twit.tv/floss36 (Podcast interview all about CouchDB)