Star Trek takes its first high concept, a double role for William Shatner, and does it so poorly, the evil twin idea doesn't return again until Star Trek TNG. The problem is that, contrary to Mr. Spock's tactless rhetorical question to Yeoman Rand near the end, neither the meek, indecisive Kirk or the violent, rapist Kirk are particularly interesting (or pleasant) to watch. Beneath the creative cinematography and cunning direction by Sean Penn's dad, used both to highlight the differences between the Kirks and to allow William Shatner to play both parts in seemingly the same shot, the character examination only proves that good is boring and evil is ugly thanks to lazy writing and Shatner's overacting. It's ironic that Shatner has cited this episode as his favorite, saying the only thing better than one of him is two of him (which is all really just a joke he liked to employ on the convention circuit) when the truth is the one character missing from the episode is the Captain Kirk we know and love.. an inescapable void.
Perhaps it's too bad Mr. Spock isn't divided instead, especially with Nimoy later stealing the show with evil Spock in "Mirror Mirror". (Another more compelling direction would be dividing Spock into his Vulcan and human halves, but this idea might have be too much for a character and series still getting its feet wet; Star Trek would finally do just such with B'elanna Torres in Voyager's first season episode "Faces").
Whatever the case, Captain Kirk does deserve some credit. Obviously the events in this episode illustrate the need for the Enterprise to have some sort of shuttle to ferry crewmembers around when the transporter isn't reliable.. and it's not long before the ship gets one, complete with a shuttle bay!
Remastered: This gets just your basic makeover, with the Enterprise establishing shots upgraded to feature a more realistic ship and planet.