I have been using Kotlin in production for 2 Android apps since october 2013.

It made things simpler to write, develop and maintain.

One big downside, as of 0.12.1218, is that builds on Android are quite slow (I got 5min builds). But the Kotlin team is aware of the issue and working on it.

Once the kotlin builds on Android get fast enough, it will blow everything else out of the water.

If Google were to switch from Java to Kotlin for android, I wonder how Oracle would feel...