Can we compare Cassandra to Terracotta ?
I used Terracotta on a previous project and you can think of it as a persisted/fault tolerant cache.
The free version doesn't come with sharding, so your data has to fit on one machine.

Other people I know have used it as a persistence layer (i.e. replacing something like an RDBMS/Cassandra/HBase/CoucheDB/etc.) and it failed miserably.