What are the use cases for non relational datastores?
In some cases that are simply nice. ZODB is a Python-only object database, that is so well-integrated with Python that you can simply forget that it's there. You don't have to bother about it, most of the time.
I'm looking at using CouchDB for one project and the GAE app engine datastore in the other.  For relational stuff I tend to use postgres, although I much prefer an ORM.  
Anyway, what use cases suit non relational datastores best?