I am looking at CouchDB, which has a number of appealing features over relational databases including:

• intuitive REST/HTTP interface
• easy replication
• data stored as documents, rather than normalised tables

I appreciate that this is not a mature product so should be adopted with caution, but am wondering whether it is actually a viable replacement for an RDBMS (in spite of the intro page saying otherwise - http://couchdb.apache.org/docs/intro.html).

1. Under what circumstances would CouchDB be a better choice of database than an RDBMS (e.g. MySQL), e.g. in terms of scalability, design + development time, reliability and maintenance.
2. Are there still cases where an RDBMS is still clearly the right choice?
3. Is this an either-or choice, or is a hybrid solution more likely to emerge as best practice?