Just realised I never said anything about "The Changeling" and "Mirror, Mirror".
The Changeling - Not too much to say. It's decent. It ends with the "lead the machine into a logic-trap" resolution, but I can forgive that because at the time it would have been fairly original. Uhura's "recovery" was utter bollocks though.
Mirror, Mirror - Great fun. A chance for the cast to completely ham-things up, which they do wonderfully. We won't see the Mirror universe again for a very long time, but it's always fun when we do. It's also got Spock's awesome goatee.