Yeah, they're all pretty early adopter, although worth it if you're feeling enough pain from data size and/or volume. A couple recent articles on Cassandra:

http://news.ycombinator.com/item?id=813528 http://news.ycombinator.com/item?id=813919

(I work for Rackspace on the Cassandra project, and we are in the early stages of deploying Cassandra internally too.)