I find Mongodb works well (OLTP) when you use it as a powerful Key-Value store with secondary indices. You'll get easy replication/failover (rolling updates of

I find Mongodb works well (OLTP) when you use it as a powerful Key-Value store with secondary indices. You'll get easy replication/failover (rolling updates of the DB server are easy), ease of use (with node.js for example) and very decent performance... Good for an application where you need frequent access to a "row" except that row is complex, with subobjects, sets and arrays (in this use case, a relational model performs much worse because of joins) Not so good for an application where you need frequent access to lists of tabular data and list/details access (where relational models shine) Your mileage may vary. Some use it for logging, for analytics, for which I have no need or experience. GridFS also works very well (but I use nginx as a cache in front of it).