I have used it for a small amount of production code in a mixed Java / Groovy codebase coded using Intellij and Kotlin version 0.11.

Kotlin worked well - clear concise code, no null worries and no cast worries.

It is peculiarly refreshing to know that my code won't even compile if I try to call a method on null and that Intellij will underline the code in red and suggest a fix.

At the time I was using it I would say the quality of some of the code in the Kotlin API was questionable - specifically the jdbc code. I used the parts that seemed sensible and wrote additional code to provide the functionality I needed. I don't think that code is even in the standard API anymore.