I've recently started using RE. I had keyed all of my tracks by hand before, and "just for fun" I let RE key them all. I then looked hard at all the tracks that it keyed one way, and I had keyed another.

There were cases where I keyed them wrong (nearly always in a compatible key, usually because I was just going too fast and not checking carefully).

There were also cases where it was off by a 3rd or 5th, and cases where it was off entirely (though not many).

So now my thing is, I key them by hand. Then let RE key them, and compare. Makes me feel MUCH more confident in the accuracy of my key annotations.