&gt;How well does Kotlin perform on Android?  Can you use all the native stuff?<p>Not only is Kotlin a great language, it is also a good fit for Android since it outputs bytecode and is entirely interoperable with java.  
The only cost of using Kotlin are the 6k methods that it adds to the apk.   You can call all the native Android APIs. Of course they are not idiomatic kotlin code, but you can cope with that, especially since the Kotlin team has written some Extensions methods in order to facilitate this process.<p>&gt;Are there some successful apps written in Kotlin?<p>I know that Expedia uses some Kotlin for its new features (since there is full interop, you can switch gradually).  
I am working alongside 12 or so other engineers on a successful Android app. Most of us (especially the more experimented) would like to switch to Kotlin but we get a lot of resistance from an extremely change averse team leader (we have just convinced him to start using EventBus and are still pushing for Dagger 2 &amp; Rx).<p>This kind of move is always complex for a large team though. You need to get everyone on board and up to speed with this new language. Even though I think that kotlin is very easy to learn (especially for a java dev), this is still a significant effort.