I've played with it briefly, and found it okay. It didn't really fit my needs, but the code quality (on a cursory audit) seemed reasonable; it looked to be heavily leveraging Akka to do a lot of the hard work. Not a criticism--Akka is awesome--but it means they have generally less code that they have to worry about directly.
http://spark-project.org/
Has anybody tried spark out? What did you think? I'm kind of afraid of using it for production because it is so new.