Anyone who wants to leverage MongoDB without the massive headache of learning all the intricacies yourself (in addition to actually running and developing your startup) should check them out, we've been using them for a long time for all of our player data like leaderboards and user created levels, can't recommend them enough.
We have 6 servers, a bunch of software spread across them, an immense volume of data arriving every second of every day, and a couple thousand users to take care of. It is really nice to "just use" MongoDB and enjoy all the benefits of it without adding their job to my own.
I often wonder why the hosted Mongo services aren't offering dedicated servers yet. It seems like an easy opportunity to provide service, automation, and monitoring value add at a high price point. Is the thinking that most customers who would opt for the dedicated instance would just run it themselves to save money?
Been using their service for my app and have been completely happy with them. Great support also.