Cassandra is just an awesome piece of software. I hope that anyone who has not at least played with .7 to go to http://cassandra.apache.org right now, download it, and follow the quick start guide.
Nice writeup by Jonathan and he has offered useful comments to me in the past (on my blog). I have not yet used Cassandra on any customers' jobs but I experiment with it (mostly in Ruby). I found the book "Cassandra, The Definitive Guide" useful, BTW.