Except it's not because:

1) It has differences in behavior with certain classes and is not a drop-in replacement.

2) It always compiles, so it's kind of slow to compile-test

It's not a 100% compatible replacement, but I've ported a few things with only trivial chances. I didn't say it's a drop in, just that it's a fine choice.

Compile/test time is ok. It's a few extra seconds to run tests, but hasn't been an issue in practice for me.

I've heard good things, yeah :)

I've tend to have found Kotlin to be the direction I'm more happy going with. It speaks to my particular itches for me personally, more effectively. I can absolutely see how it's a very effective choice.