I just got jmxtrans set up with Kafka 0.8 over at Wikimedia. We're using puppet, and the https://github.com/wikimedia/puppet-kafka module to install and set up Kafka. This module now comes with a kafka::server::jmxtrans class which will automatically set up jmxtrans with some nice Kafka queries.
If you aren't using puppet, you can use the rendered example jmxtrans JSON config here:
The exact queries will probably change as we get more operational experience and think of more things we want to monitor.
Hope this is useful to someone!