We're using Kotlin with our main back-end project. It's initially Java8 Spring Boot, the project was around 1M LOC when Kotlin was introduced, we've never looked back, all new code is now written in Kotlin, we're also migrating Java code to Kt when we're changing something big enough to warrant the conversion, we've also migrated some part of gradle build scripts to Gradle kotlin-dsl. It's awesome.
Suggestion, introduce no-arg and allopen plugins from the start - will save you some frustration 🙂