i've started using Riak and i've fallen in love. I still love couch but my new db of choice is Riak, for both Erlang and Rails projects.

My recommendations (to my friends) on noSQL: 1. If you're doing replication with devices, or the replication endpoints are not in your control, use CouchDB. 2. If you need speed, and you don't mind losing your data, use mongoDB. 3. Otherwise, use Riak.