This is a game changer. I've used it over the last few 3 or 4 months, and it is really stable and usable. I am betting an entire (although academic) project on it: http://proofpeer.net (note that Clojure is mentioned there as a language of choice, but I switched to Scala since then as Scala provides a similar level of conciseness with all the goodies of a powerful static type system with support for both Java and Javascript).