It's cool to hate MongoDB, it used to be really cool to like MongoDB. Same thing happend with MySQL probably 10 years ago. I have built some apps with Mongo and it's fine as long as you don't expect it to be a SQL database. A lot of times I wish it had features x,y,z from sql land, and other times I wish MySQL was schemaless.

Nowadays, I have begun to defer my database choice until I absolutely have to put an app into production if possible. That way I can write an app and have it working without worrying too much about queries, caching, and so on until I need to actually put it out on a real server.

MySQL, Mongo, HBase, Cassandra, Postgres, Riak, Redis, Memcache, Couchbase, Filesystem? It doesn't matter.

They're all going to break at different times for different reasons depending on how you use them and what scale you are at. Everything breaks at scale eventually.

Build something awesome enough that it breaks under the user load. That's a good problem to have.