A stellar plot, superior dialogue, and non-stop action ranks this episode among the best. It also raises again the question so often posed by Trek: how would my evil counterpart manifest itself? Or for some people, perhaps the question is actually "Am I the evil counterpart, living out a life of brute selfishness while my merciful and compassionate nature lies dormant - or existing elsewhere?"
In addition to the philosophy and excitement generated by the nature of this episode, there are plenty of "gotta love-its" in this episode: from Kirk's fringed, sparkly-gold disco tunic to the hilarious fight scene in sickbay in which Uhura hurls herself flailing into a wall and Kirk smashes a paper-mache bust over Spock's head. There's the alternate Sulu, who with his lazy-eyed, maniacal smile is every bit as creepy as the ordinary Sulu we've come to know and dread - the chief difference is the jagged scar running like a topographical mountain chain down his porous face. Evil Chekov finally has manageable hair, but his grin is a frightening combination of an over-eager puppy and a sadist. The audience is also treated to a glimpse of all of his fillings when he's in the torture chamber. Then there's the whole behavior of the alternate universe personnel, beginning with our introduction to them through the goateed Mr. Spock and the poor Mr. Kyle, who bumbles and bumbles in both worlds. "Your agonizer, please, Mr. Kyle." Apparently, crew are equipped with their own torture devices used against them in order to teach a handy little lesson - fantastic!
Then, there's Kirk. Amazing the oh-so-subtle differences in him when he's "good boy Kirk" and when he's "bad boy Kirk" - we've seen this theme several times throughout the series, but either way you slice him, Kirk's a womanizer and a cad in both roles. I especially like the dialogue between him and McCoy, which runs like this:
McCoy: Captain, what do you suppose our counterparts are doing back on the enterprise?
Kirk: Well, I know mine must be engaged in some particularly bad acting and effected emoting, trying extra hard to play the part of the villain that Shatner in all his skeeziness lends naturally.
Then, of course, the scene switches to the evil Kirk screaming and having a fit before being thrown bodily into the brig - hilarious!
Then there's poor Marlena, who is stuck with Kirk in both universes.
Marlena: [after Kirk plants one of his smash-jaw, pursed lip kisses on her] It's been a long time since you kissed me like that. [you mean...badly?]
Of course, the writers had to add some frat boy humor at the end on the bridge, which took away from the main theme of the episode, in my opinion, because really, we're just once again seeing Kirk at his worst:
Kirk: [undressing Lt. Moreau with his eyes] She just seems like a nice, likeable girl. I think we could be friends....and, I've seen what she looks like in another universe in nothing but her underwear and a see-through, rainbow-colored robe!