This is another of those third season episodes that belies the generally held opinion that the third season is garbage. Yes, it's what they call "high concept": "What if Kirk went to the Gunfight at the O.K. Corral?"
You get another alien/alien race in the third season, though. The Melkots sharing some mild similarities with the Medusans, Excalbians, and Vians. This is another race that is unfamiliar with humanity and are really "alien." You can't really say the idea of executing someone with their memories is dumb because... well, that's human thinking. The Melkot have their own alien ideas about how to dispose of intruders and that's what they do. They're not interested in experimenting on humans or getting along: they just want to get rid of them and move along with their lives.
There's the usual clever Gene L. Coon dialogue along the way, and the episode is filled with all kinds of cute little quotes and asides like McCoy and Holiday as fellow doctors, and Chekov flirting with a local even in the middle of an execution. The sets add an appropriate surreal touch to the whole thing and neatly goes from a general sense of the unureal, to the cold hard fact of Chekov's death, to Spock figuring out the whole thing. Spock saving the day third-season style is a bit irritating, as this is another of those "Spock can do anything" third season plot resolutions. Still, the guest cast has fun with this mini-Western, and Kelley no doubt had fun comparing it to his own appearances in a Earp/Clanton movie years earlier.
So overall a good episode to watch, and it stands up well in any season. It's just regrettably one of those that gets buried in with some of the third season muck.