We've been using Cassandra for some months now and have a cluster in production.
To help us learn more about Cassandra and meet other people who are using it I have just launched the London meetup (partly prompted by the guys who launched the Israel and Bay Area groups recently).
If you're in London and are interested in Cassandra then please sign up!
The aim is to meet informally once per month; hopefully with a short talk (tba). The venue will be somewhere central (around Soho, again tba).