We’re using it at my current company. We have over 100,000 lines of Java and have made the switch to doing all new development in Kotlin. It’s interoperability makes it easy to have a mixed project. After 9 months about 10% of our code base is now Kotlin. Since we made the switch we’ve reduced our crash rate and the maintainability of our code has increased. If you’re waiting for Kotlin to overtake Java as a language before you switch, it will never happen.