One of the stories in which the deciding factor for using Cassandra was primarily the data model and not its scalability characteristics:
We started working with relational databases, and began building things primarily with PostgreSQL at first. But dealing with the kind of data that we do, the data model just wasn’t appropriate. We started with Cassandra in the beginning to solve one problem: we needed to persist large vector data that was updated frequently from many different sources. RDBMS’s just don’t do that very well, and the performance is really terrible for fast read operations. By contrast, Cassandra stores that type of data exceptionally well and the performance is fantastic. We went on from there and just decided to store everything in Cassandra.
Original title and link: Cassandra at Workware Systems: Data Model FTW ( ©myNoSQL)