Not sure of the state of Scala on Android, but having a language that actually has real closures is appealing. To say nothing of ADTs, nullability, etc...
From my understanding it isn't that great. But Kotlin works well and it's far better at those things than Java. Theres also Clojure but I don't know how well supported it is. Running both of those on iOS is also possible via RoboVM - no need to wait for a good Swift on Android solution.
are you using Kotlin personally ? I have heard a lot of good things about Kotlin (especially the fact that Kotlin code seamlessly works with existing java code). But not sure if I should wait till 1.0
I haven't for any real projects, I've only played around with it. But it seems like there's a decent number of people using it today, so my guess would be that waiting isn't necessary. If I were working on a cross platform mobile/web app today, I'd probably choose Kotlin.