I'm working on a system that's using Cassandra on a small scale (like, probably less than a million entities) now, it's great. Haven't had any consistency issues because we've designed our data flow properly (SQL will help you paper over a bad data flow for longer but that's a double-edged sword), and getting true just-works HA (master-master where a failover doesn't interrupt your writes) out of the box is a huge advantage over something like Postgres.