Besides the answers given above about when to use and when not to use Cassandra, if you do decide to use Cassandra you may want to consider not using Cassandra itself, but one of the its many cousins out there.
Some answers above already pointed to various "NoSQL" systems which share many properties with Cassandra, with some small or large differences, and may be better than Cassandra itself for your specific needs.
Additionally, recently (several years after this question was originally asked), a Cassandra clone called Scylla (see https://en.wikipedia.org/wiki/Scylla_(database)) was released. Scylla is an open-source re-implementation of Cassandra in C++, which claims to have significantly higher throughput and lower latencies than the original Java Cassandra, while being mostly compatible with it (in features, APIs, and file formats). So if you're already considering Cassandra, you may want to consider Scylla as well.
