I think that this is a great project, and would be exactly what I need for ProofPeer (http://www.proofpeer.net) because I have lot's of code that needs to run both on the server and in the browser. I am using Clojure/Clojurescript currently and am pretty happy with it. But I am missing the possibility of defining abstract datatypes in Clojure, so my first choice would have been Scala if Scala.js would already be mature and reasonably production ready.