MongoDB really is simple to use. Unlike CouchDB, you can make adhoc queries without defining map/reduce functions. In MongoDB, array elements are indexed so searching for words, tags, etc. is easy. The Ruby client support is excellent. Casandra is really easy to set up (installing the cassandra gem, then doing 'cassandra_helper cassandra' installs all cassandra dependencies). But Casandra requires a lot of application specific configuration. I always leave MongoDB running on my MacBook and two of my servers so it is ready at hand.