Is mongodb really a good solution for storing things? IIRC one concern about coinbase (YC) is that they used mongodb for storing financial information.
We like to embrace different technologies and use each for its strength. MongoDB is excellent for a whole class of use cases and we like it a lot, but we also use other persistence solutions as well. Check out Polyglot persistence post by Martin Fowler, it's an awesome read. http://martinfowler.com/bliki/PolyglotPersistence.html
Stripe uses mongodb, if its good enough for them, its probably fine for financial info. As I recall, there is some aspect of mongodb that lets it scale up without maintainance, making it good for a service that just can't go down.