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.
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