I've done some work with it just recently. Happy to answer any questions, though I am not an expert. Overall, I think it fits very well in the Swift space and the tooling is clean. I wrote a sample DLL in it yesterday to become more familiar with it and work through some of the FFI challenges [0]. I have found it much easier to use than Scala Native (especially if you're a Windows person).

0 - https://github.com/cretz/kotlin-native-sample-agent/