One thing to note in my experience with Mongo vs Memcached for certain tasks. Back in the day we used to use MySQL as a shared session storage for load balanced web servers - this is obviously something from the past and since then we moved it to Memcached. Once we started implementing Mongo, it seemed to make perfect sense to just use Mongo instead of Memcached to store that information - but to my surprise (and I love Mongo), Memcache outperformed Mongo as far as overhead and latency goes for managing the sessions. We now keep all session management in Memcached at all times and Mongo for everything else.