Briefly. There are Kotlin areas in the codebases I work on, particularly in Android code. It's not something I've needed to use yet, but I've heard good things and I'm interested to get into it once we start using it more.
Strongly second the GP. I ramped up over the past year and I compare it to C# or TypeScript as far as the type safety and async features go. That whole time I kept running into QoL features and sugar that were just a joy to use.
The best side benefit for our Java heavy org was that the migration risk is very low due to excellent JVM/Java integration.
I started using Kotlin in Java 8 before the "record" keyword existed to be able to use "data class" instances. This provided very nice semantics for defining immutable structures, and the resulting classes could be imported into the existing Java codebase.
As the project progressed we started adding more in Kotlin and it massively improved developer experience.