I don't really have a strong opinion in the game but mind you that MongoDB is pretty darn fast http://www.peterbe.com/plog/fastestdb At least when you're not at scale.

And it's pretty cool that you can make a choice between fast writes or safe writes. You can't have both but at least you can have the choice.

Having said all that, I generally prefer Postgres in almost every possible case.

However, this RethinkDB project looks sexy and with a great potential.