I'm a pretty happy camper using Postgres with hstore: http://www.postgresql.org/docs/9.1/static/hstore.html

You have the flexibility to store arbitrary JSON blobs when you need to, the stability, maturity, and performance of Postgres, and the ability to migrate data to a more rigid schema once your project matures to the point where data validation is more important than raw prototyping speed.

The main drawback so far is that the query interface is a little clumsy.