Fortunately for me Kotlin it is a Swiss Army Knife in Java world, and you are right, it’s not free, same as RetroLambda Kotlin come with its price. I do not recommend to use RetroLambda because of the price it’s too high.
What you’ll get for your project with RetroLambda? You’ll write less code in your project! That’s it. But truth is — Android Studio folds lambdas so good you just don’t see difference between real lambdas and anonymous classes in IDE.
What is the downside? You still cannot use RetroLambda library projects. False warnings for static code analysis. Each update of any plugin could break your build. I’m now sure if it works with Instant Run. https://github.com/evant/gradle-retrolambda/issues
https://github.com/orfjackal/retrolambda/issues Still 60> opened issues, for the large project you’ll face at least few of them for sure and it will consume a lot of time.

You still could use RetroLambda for small home projects, but Jack would be a better solution in this case.

Let me know if you’ll have more specific questions.