curl -L http://debian.datastax.com/debian/repo_key | sudo apt-key add -
sudo sh -c 'echo "deb http://debian.datastax.com/community/ stable main" >> /etc/apt/sources.list.d/datastax.list'

sudo apt-get update
sudo apt-get install cassandra

Really wish there was a cassandra-core and cassandra-server divide as more often than not I just need the c* tools and libraries and not so much the server itself.

Also 1.2.5 is TOXIC as all hell for high read/write environments.