mongodb is the best database in the whole wide world at the moment. I encourage everyone to jump in mongodb for agile web scale development with full big data capability.
I'm sure that it is a viable choice for some use cases, it's just that I didn't found a use case for it yet.
Being able to choose from PostgreSQL, Redis, Cassandra, heck, even ElasticSearch made me always choose one of those over MongoDB, at least for the problems which I had been trying to solve.