How does it affect built time and the feedback cycle?
At the moment the gradle plugin doesn't do incremental compilation. If you use it for Android, that's an issue because of the Android build system.
I use Kotlin for free standing programs that run on my laptop/desktop. In these cases it's about as fast as Java, but that's because IntelliJ controls the compilation itself and gradle/maven is used only rarely.
I personally use Kotlin on a daily basis for Android development and build time is comparable to the one of Java. May I also recommend Anvil framework (https://github.com/zserge/anvil)? It brings declarative layouts and data bindings, which works best with the immutable data types of Kotlin (much like React does in the web).