I have been using MongoDB every once in a while and I think it is great. The C# driver has linq support which increases productivity to the roof. However, the feature I miss the most while using MongoDB is multi document transaction support. Their site provides a workaround[1] that from my point of view is not worth implementing. Instead I prefer to switch back to traditional SQL databases in those situations.

[1] http://docs.mongodb.org/manual/tutorial/perform-two-phase-co...