One of our ex developers was a big kotlin fan and enabled it in every java/maven project he touched -that is actually quite easy and interoperability between java and kotlin is fine. He also refactored every class he touched to kotlin. After he left, no one else was really into continuing his work. Our mobile devs use kotlin though and they are happy.

I'd say it's nice and I would actually start every new (spring/java) project I do in kotlin now. But for existing projects, I actually see no value. It does not magically make code easier to understand and it doesn't reduce bugs, it's just a bit more fun and challenging to write and pads your resume...

We did have one issue where upgrading the kotlin version from 1.x to 2.x in a project broke all depending projects due to a classfile version issue though. I deferred it by just forcing the version back to 1.x. That's the kind of annoyance you can encounter.

I know one of these developers as well! It's me :D