This review contains minor spoilers.
This episode has 'average' written all over it. I would not say it is as bad as some others consider it, but even so, it is far from a standout example of the Original Series.
The plot of the story in many ways feels like a first season episode, revolving around Kirk and Spock; McCoy takes a back seat for this one.
At first, I thought it was going to be a simple retread of the first season's "Dagger of the Mind". Although there are some similarities (both set in insane asylums, and both featuring torture chairs – which was in fact the same prop used for both instances), thankfully there are enough differences between the two episodes to prevent it from feeling like a complete rehash.
Steve Ihnat gives a reasonable performance as the insane Garth, but I felt that the writing of the character let him down somewhat. We are given vague ramblings of how he plans to become "master of the universe" (though to be fair, he is insane!), but I felt the writing for the character and his plans deserved to be sharper.
Orion slave girl Marta is played by Yvonne Craig, who is probably best known for playing 'Batgirl' in the third season of 1960s 'Batman'. Craig plays the part very well, and makes Marta very sexy, even if she IS green!
It is very unexpected that the character is killed off later in the story, especially in such a callous manner.
As with some other reviewers, I questioned that Spock did not find a more 'logical' way to tell Captain Kirk and Garth, posing as Kirk, apart at the climax of the story.
The editing, though, during the final fight between Kirk and fake-Kirk Garth, was quite good, making William Shatner's double not appear TOO obvious throughout.
Here in the U.K., this episode was skipped in the BBC's run of the series (one of several episodes not to be shown by them for various reasons), until it was finally included in the 1990s. Apparently the reason was due to the torture chair sequences. I was quite surprised at this, as I personally didn't find them as graphic as some scenes in other episodes; and at the very least, they would have been easy to edit around the offending moments.
All-in-all, to paraphrase myself from the start of this review, this episode is watchable, but decidedly average. Not terrible, but not outstanding either.