[ptnapoleon]

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.