I've used Kotlin for standalone Java programs, it's great. According to Jetbrains, it's roughly 50%:50% between Android guys and Java guys. I guess, after yesterday announce it'll shift to Android.

For example Spring announced that Kotlin will be first class citizen with next major release. So things are definitely bright on the server side.

I've been doing few projects with it and find it very lovely. Interop with Java works flawlessly in my experiences and RxKotlin extending RxJava makes that side work nicely as well. For one project[0] I used Ratpack as the web tier and that with Guice locked in place very easily. DB side of things I have used Kotlinquery which did its job nicely for my small use case.

In another I used Spring Boot where Kotlin felt like a native in that environment.

I'm guessing if you are building something with ORMs or similar where Java Bean convention is needed, you might lack the beauty of immutability and data classes (there are workarounds though) but other than that most pieces drop in place without a hitch.

0: https://github.com/Xantier/trycatch

Yes. I lead a team that's writing a new piece of what can be thought of as enterprise middleware in Kotlin:

https://github.com/corda/corda

It's a distributed ledger/blockchain system being written for large, conservative banks and is around ~90%+ Kotlin code.

There's not much to say about it, it works fine, I wrote about it here:

https://www.reddit.com/r/programming/comments/6bqo7n/kotlin_...

I've used it to write a real time HN clone using GRPC. It was a wonderful experience. The Java interop is no joke, it works flawlessly.

When you have the option to use Java 8 without caveats (unlike in Android), the benefits of Kotlin aren't really _that_ visible.

I'm never going back to Java on Android though.