This episode suffers straight away from being yet another 'parallel Earth' story. We had already had "A Piece of the Action", "Patterns of Force", and the weak "The Omega Glory" (as well as "Miri" in the first season), and here we get another such tale.
But it's a shame that this episode comes so close to three of those other examples, as in its own right it is a very good story.
Indeed the parallel earth concept had been done a number of times before by this point, but here it is at least given some depth and background. The crew puzzle over it and discuss it and debate over it, whereas on some other occasions (think "Miri" in particular) they barely blink an eyelid, and it is never really explained.
The episode also features another "Starship Captain gone bad". Again, there are similarities, coming so close after "The Omega Glory", but here it is handled with more care, and more believable.
The "forced to fight in gladiatorial bouts" idea had already been used in "The Gamesters of Triskelion" (produced after this episode, but broadcast before), but this episode does it so much better.
The episode also features one of the best scenes between McCoy and Spock, and serves as a great showcase of their strange relationship.
Similarities aside, this is by far one of the better episodes of the later second season. By this point, the series had started to lose its shine, but episodes such as this still make it a great watch.
I think it could have been a 100% series classic, but the pacing needed tightening up a bit. The final act sags slightly, and the whole Drusilla 'Kirk's personal slave girl' element was not needed. As has already been commented on in other reviews of the episode, would Kirk really lay back and sleep the day away while Spock and McCoy are held prisoner and awaiting execution?!
All-in-all, one of the best episodes from the end of the second season.