For me there are two definite use cases for document DBs.
- Prototype apps
- Anything where you would use an entity-attribute-value model.
I've mentioned before that when I first started looking at MongoDB I went to a presentation from 10gen talking about how data modelling was MORE important with NOSQL, not less. 10gen became MongoDb inc so that is straight from the horses mouth.
I'm not worried about the tech (although it is still quite immature). What concerns me is pages that list MongoDB ORMS and ODMS.
Perhaps I'm missing something as I thought one of the key benefits (from a developer's perspective) was that you didn't need an ORM because a document database made things so much simpler.