When will I be able to play with this?

Right now. See the samples README for details on how to get started.

How do I set up my development environment to play with this?

See the set up dev environment section in the samples README for instructions.

What's the overall roadmap?

1.0 M1 has been released, and a subsequent milestone release of Gradle Script Kotlin will ship with the forthcoming Gradle 3.0. We'll be posting further information about the roadmap to Gradle Script Kotlin 1.0 GA soon.

Is using Groovy for my build scripts deprecated?

No. Gradle's Groovy support is not deprecated, and will continue to be supported.

Will existing plugins still work when I write my build logic in Kotlin?

Yes, they will continue to work without a problem. Gradle plugins can be developed in any JVM language and they interoperate well with each other as well as with build scripts written in Kotlin or Groovy.

Do I have to use IntelliJ IDEA when using Kotlin for Gradle?

No. Although JetBrains is the company behind IDEA and also the inventor and driving force behind Kotlin, JetBrains is also committed to providing Kotlin support for Eclipse. The IDE support for writing Gradle build logic in Eclipse will actually improve with Kotlin once we integrate the Eclipse Kotlin support into Buildship, the Gradle plugin for Eclipse.

In what language should I develop my plugins?

You can develop your plugins in any JVM language, but as part of this effort, we are working on making Kotlin the language of choice for developing Gradle plugins. Stay tuned!