Kotlin is positively awesome, but when I tried it (admittedly last year) it didn't seem ready for production yet. It worked all well as long as stuff was kept simple, then I used Realm.io, wrote some unit tests and got all sorts of NoClassDefFoundError exceptions, dexmaker errors etc. Long story short, I couldn't fix it. As I said, it's been a while. I hope things have been improving and continue to improve. The language itself is superb
I've just been playing with it some in the last couple of weeks. I get the impression that they're still iterating pretty rapidly on it - at least partly because the official practice repo uses trait all over the place, which they apparently deprecated and changed to interface.
I will say that it It looks very promising so far, and seems to have the best of C# and Ruby, assuming you're sticking with compile-time type checking, plus a few more tricks besides. I'm not that up on JVM languages though, so I'm not sure that it's definitively better than any of the other choices.
