I initially used Kotlin for a project at work, and I would agree that it is underwhelming. Pivotal had announced official support for Kotlin with Spring Boot, which is what motivated me to give it a try. However, there was no support for the @Transactional annotation in Kotlin at the time, so I ended up writing a decent amount of Java anyways. Then there was a compatibility issue with one of our internal libraries, and I had to rewrite all of the controllers in Java. By the time the project was finished, the majority of the code was in Java, and I hadn't really gotten any benefit out of the Kotlin, so I decided to just use Lombok and replace all of the Kotlin code with Java. My builds got about 3 times faster after that because Kotlin seems to have performance issues with Gradle.
The 1.1 release is promising many features that make me want to revisit it, though: https://blog.jetbrains.com/kotlin/2016/04/kotlin-post-1-0-ro...