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.