I don't have anything with Kotlin in production. But locally, I'm working on a kotlin-common project (JVM+JS) and that's going well.
There's not much browser integration, it's mostly isolated logic some a big of platform specific stuff. For that limited use case, I've not had any problems. It's stable and expressive and indeed, much better than Java.
I do get the impression JVM-Kotlin is much more popular and easier to find information for.
TLDR; The language is great, the JS backend works, but I don't know much about dom manipulation or integration with js libraries (other than qunit).