We forked our app to give it a try. It's definitely an interesting contender, but in our case, we didn't see any significant performance gains and so it wasn't worth moving away from MySQL. It did help us simplify our MySQL schema a lot though and i wouldn't be surprised if we revisit it when it hits v1. For someone getting started with it, the google group is quite useful [http://groups.google.com/group/mongodb-user] and the project team was pretty responsive.