I was involved in a migration from MongoDB to Cassandra at my last job.

We switched to Cassandra because of the control it offers over how your data is laid out on disk, and it's ability to not totally fall apart under load (in fact performing really well). It's also a lot easier to deal from an ops perspective.

I did a post about the migration here: http://blakeeggleston.com/migrating-databases-with-zero-down...