That is really exciting! I am going to take a serious look at jsonb and using Postgres now, although FoundationDB has caught my eye with its table group functionality: https://foundationdb.com/layers/sql/documentation/Concepts/t...
Do bear in mind that you'll never get MongoDB-style in-place updates in PostgreSQL, due to MVCC. You may save a round-trip with the entire JSON object once they implement update operators, though.
[1] http://en.wikipedia.org/wiki/Hierarchical_database_model