This happens to be an oldie but a goody. On the same topic, I might as well spread the wealth of links I've amassed for learning and understanding Cassandra:
Coincidentally, I came across this article on my own last weekend. Definitely a great read if you're trying to learn Cassandra, though it doesn't explain why you should learn it.
Does anyone know of a resource that clearly explains the advantages and disadvantages of the various RDBMS alternatives?
Up and Running With Cassandra http://blog.evanweaver.com/articles/2009/07/06/up-and-runnin...
Building A Small Cassandra Cluster http://posulliv.com/?p=149
Cassandra PHP CRUD Class (Pandra) http://github.com/mjpearson/Pandra
Getting Started - CassandraWiki http://wiki.apache.org/cassandra/GettingStarted
Cassandra In Action http://spyced.blogspot.com/2010/03/cassandra-in-action.html