I was tempted to do some comparison, especially to Couch/Mongo/Cassandra, but ran out of time. I'll consider a follow up that compares them.

In short, Riak wins out to me against all three:

* Faster than Couch & easier clustering * Better data integrity than Mongo * Less conceptual overhead & simpler code than Cassandra

Riak is distributed (it automatically shards data on many servers). CouchDB runs on a single server, although it can easily have as many mirrors as needed.

Also, Riak features an integrated full-text search engine.

Why does there need to be?