I'm glad to see Kotlin making its proper debut. Android has been crying out for something a little more expressive and less verbose than Java, and Kotlin fits the bill perfectly.
Kotlin is super nice. As a former C# dev who went into Android, I was really dissatisfied with Java, and Kotlin was my rescue. I had to write some C# recently, and I have to admit Kotlin does feel nicer. (Admittedly it is still lacking some good stuff in comparison, eg. _yield return_).