I've been experimenting a bit with RethinkDB within the last few days. Its stability is definitely not there (too be fair, they've made this clear), so it's crashed when I needed it most. But here are the finer points:

1) Their team is extremely responsive. In fact, the most responsive team that I've dealt with on Github. I've posted at least 4 Github issues during various times of the day and evening and I've gotten a response in less than 10 minutes. Most of the time, more than one member responded.

2) They are not over selling their product like MongoDB did. They are very clear about what their product does and doesn't do.

3) Clustering is stupidly simple. You can have one setup within minutes and have your servers geographically located throughout the world.

4) The web UI makes administering the databases easy and fun.

After they get the kinks (stability & docs) fixed, this database will definitely have a bright future.