I attended a user group meeting in which we had two speakers talk about and show some examples of both Cassandra and MongoDB. Let me tell you I was not impressed! I wouldn't touch that stuff with a 100 foot pole. The documentation is almost non-existent, it's hard to keep up with the revision changes and what seemingly basic features are in what version, and I just don't see the cost benefit in trying to use it.

I asked one of the speakers what the largest database and system he was using Cassandra for and he said that he hadn't even used it in a Production environment yet and that in the development environment it was just running a very small database (a few hundred megabytes) on a single server.

Oh, so add one more item to my list of complaints above. If you run into some problems, who's going to help you? Chances are, there won't be anyone else who has done what you're trying to do.