I've used C* (36 nodes/20Tb) and it was pleasure to manage it even in the early versions. At the time of picking the db (3 years ago) Riak was slower as they only had http available so C* won by a large margin.
One case I thought Riak can be a good choice for is for quick prototyping / interacting directly from web clients (as it has neat http API).
One case I thought Riak can be a good choice for is for quick prototyping / interacting directly from web clients (as it has neat http API).