MongoHQ have a pretty awesome product. The only concern I have with them is I noticed they recently capped databases to 20 gig but that's 3x more than my leaderboards come to at the moment so that's future-me's problem.
The custom data on the leaderboards and levels is just ridiculously easy, it's basically just copying object properties/values on the user end straight in to document properties/values in MongoDB.
Future-you should fear not. The 20gb limit is a soft limit and we have many who have gone past it. It is really there for two reasons ...
1) To give people some reasonable expectations about how much data they should be putting on the plan given it is a multi-tenant server and they are sharing resources.
2) To make room for an expanded offering, both a larger high-availability shared plan, and a dedicated server plan.
The custom data on the leaderboards and levels is just ridiculously easy, it's basically just copying object properties/values on the user end straight in to document properties/values in MongoDB.