Kotlin M12 is out
blog.jetbrains.com
blog.jetbrains.com
It currently runs on 0.11.91.1, but will (hopefully) soon be updated to 0.12.200 (M12). The reference documentation was already updated for M12: http://kotlinlang.org/docs/reference/.
Another problem is compiler performance (both Kotlin and Scala compilers compile less statements per second than Javac), but it's not that dramatic.
Finally, dexing languages with runtime (kotlin - ~200KB, scala - more than 1MB) takes time.
Luckily, Buck optimizes most of it.
For me, the tight Java integration and low learning curve makes Kotlin a good candidate. Also good to have a modern language for all Android versions on the market. The compiler optimizations seem to be great, and their work to support things like Dagger, which modifies code at compile time, also seems fruitful.
edit: typos