Thanks for sharing your experience. Having performed a sort of reverse migration (mongodb -> pgsql), I'd have a few questions.

Is it possible to know the use case of your setup? Which size (storage, load etc.) are you currently operating at? What is the projected growth in the medium term? What is main the benefit of this migration? Is MongoDB solving a specific problem that could not have been addressed by mysql/pgsql (or some document oriented/nosql other than mongo)? Are you using any ORM and/or middleware to interface to MongoDB?