This is far down on my list of Star Trek episodes, but I think it gets too much scorn. Actually , I think it is one of the funnier episodes of the Original series. I especially get a kick out of Charles Napier's Adam. I'm not sure if the episode was supposed to be funny, but it is. There are certainly much worse episodes- bleak and nonsensical episodes like "the Empath" or " the Paradise syndrome". If you don't take it too seriously, it is a moderately fun episode. It is also interesting as a social commentary on the times the show was being made.
It surprises me that this is the lowest rated episode of Star Trek TOS. Granted it is silly, but at least it has some laughs. The worst episodes are just boring.