I have been using RethinkDB for a while now (although mostly for small side projects + maintaining the Go driver https://github.com/dancannon/gorethink) and have really enjoyed it, having a functional query language is quite refreshing. The recent introduction of change feeds are also really cool, building a realtime app with websockets was surprisingly easy.

Honestly I would recommend RethinkDB to anybody looking to start a new (small-medium sized) project. While there are some small performance issues this is to be expected for a project at this early stage and after seeing how the RethinkDB team works I am confident that these will be sorted pretty quickly.