I just started looking into and according to a few of my sources it isn't ready for primetime due to crashing issues and other problems with linux kernel versions

The threading model is not quite decided and the compiler is currently quite slow, but these things are being worked on and I think it looks very promising.

The Objective-C interoperability doesn't fully support generics, so writing iOS apps entirely in Kotlin is inconvenient, but some companies are apparently (it was said at KotlinConf) already using it to share business logic on iOS.