Really really good for mastering, probably one of the best limiters available. Unfortunately it also eats a lot of CPU. I like to mix and master in the same DAW session, so if you have a project with already many mixing effect activated, this might overload the CPU. If you do mastering at a separate stage, it's less of an issue, since you won't have as many realtime effects activated.
I hope the great Tokyo Dawn Labs team can come up with further CPU optimizations.