If you want to use MongoDB in a project and you don't intend to rely heavily on the aggregation framework, the consider TokuMX (http://www.tokutek.com/products/tokumx-for-mongodb/) as it alleviates many of the shortcomings of MongoDB (data compression, document level locking for writes, ...) + it adds transactions.

It's a drop in replacement so it will work with current drivers. (if you have a running mongo cluster however expect quite some work if you want to migrate)

(I have no affiliation with TokuTek whatsoever except that I use their product)