Personally, I found Cassandra really convenient when I first switched from MongoDB. Though it takes some time to get used to the non-relational data structure, it allows you to be more flexible and to scale your database.
If anyone can't decide on the DBMS, check
this article out. It describes all the pros and cons of the existing solutions.
As LXer said, the main problem of Cassandra is that there are no too many admins, though if you work in a small company, it's absolutely okay to manage the system by yourself.
