So Google did some things a few days ago and released a new beta of Android Studio 2.1.x series and the N preview and the new Android Gradle plugin 2.1.x series. Should you start using Jack and Jill now?
Not, really at least in production. As the way dagger, butterknife and rest of the annotation stuff integration has not fully played out just yet. While its true that N will require Java 8, one could still get away with using the previous retrolambda tooling and still have Jack and Jill not enabled.