For me Kotlin is almost a perfect replacement for Java. It has very few drawbacks, but generally it's much better and it's a pleasure to use it. Not to mention solid IDE support. I've used Kotlin in two small projects. One project was completed year ago and now I'm improving it. Kotlin is just awesome when it comes to refactoring or generalising. Much better than Java.

I don't want to compare it to Scala, they target different auditories, IMO. I tried Scala, it's a lovely language, but it's too complex. I had to spend a month before I was able to write a quality code using Scala and I still had a lot of problems reading scala collections source code or even understand scalaz. On the other side I spent one evening reading about Kotlin (awesome and very complete, yet small documentation definitely helps) and I knew the language, could read its standard library and write production-quality code. For me that's a deal breaker.