I think Cassandra is a better fit for interactive use cases, not for reporting. Also, basically it's super heavy duty and it should start to shine when you're really serving entire Internet (on the scale of Reddit, Expedia etc.) and your Cassandra cluster is distributed across DCs across the world.
I haven't really worked in this space for a couple of years so I don't know if the cloud offerings have already completely matched Cassandra's features and robustness.
Of course, I have been totally unable to determine how they merge rows/partitions without cell timestamps. It's a black box.
I was just on a "Keyspaces" meeting where the sales dude basically described dynamo billing, dynamo provisioning, feature shortfalls obviously due to dynamo, but would not admit it was dynamo.
I haven't really worked in this space for a couple of years so I don't know if the cloud offerings have already completely matched Cassandra's features and robustness.