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...

Here are two blog posts I was able to find from companies who switched Mongo -> Cassandra:

http://www.fullcontact.com/blog/mongo-to-cassandra-migration...

http://relistan.com/cassandra-vs-mongo/

The main selling points for Cassandra over Mongo are that it scales to more machines easier for handling larger workloads, and the write performance is better.

Yes. I have switched back and forth between them before. The main advantage of Cassandra for me is that it is multi master. This makes it perfect for the cloud where you can trivially scale horizontally whether in the same AZ, across multiple or even across entire DCs with just a config change. It really is a dream to deploy and manage.

MongoDB is a document store and so if you data is structured that way it is unrivalled. But if isn't (more than likely) than it will suffer greatly as you start to increase the number of joins.

> Has anyone switched between the two?

Just anyone who has run in to scaling problems with MongoDB. ;-)