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BlueCrew is written entirely in Kotlin. 2.x was Java, and I made the switch to Kotlin for 3.x, and regret nothing. Kotlin is a breeze to code in, extension functions are a fantastic, and the whole codebase is SO much cleaner. You can convert little by little, but I just converted everything at once. A couple libraries, particularly ones that rely on annotations to generate code, can be problematic, but most of those exist to get around excessive java boilerplate, so you can do without them. Downside is longer compile time, but that's getting better.
As for libraries, we're using Kotlin Android Extensions, Retrofit2, RxJava/RxKotlin/RxAndroid, Sqlbrite, RxLifecycle, PaperParcel, and a few others.
If you're unfamiliar with PaperParcel, it's a must-have for Kotlin + Android. Makes creating parcelable data classes easymode.
I've tried Anko, which is almost awesome, but the layouts in Kotlin rather than xml is just not there yet.
Kotlin Android Extensions is a must have. It basically eliminates the need for Butterknife/Kotterknife/etc in a cleaner, more sensible way.