Kirk is caught in an age-old class war when the Enterprise visits a planet to obtain a needed mineral.
Certainly not the worst of the Season 3 "issues" episodes, there are some good and bad things going on here. The basic premise is good, class struggles between those that work and those who enjoy only leisure. The false-looking "cloud city" actually re-inforces this message, a castle on a puffy cloud is no more real than what the segment of the population that lives there represents. It's nicely symbolic. The balcony on Stratos for beaming down to the surface is a nice set, poised high in the air. Also, the matte painting looking down on the planet from Stratos is well-executed.
What doesn't work is the idea that an unseen gas is the true cause of the social disparity on the world, it's a cheap cop-out - though I do think the line that the airless cave is also an example of an invisible gas is a good piece of science fiction writing. Probably most annoying is Spock's apparent fascination with the wooden and empty-headed Droxine, it plays badly and almost seems like it is recycling cut lines from "The Enterprise Incident" ("beauty...can be...disturbing").
In contrast to the original story written by Gerrold, I like the ending here, the script is still ambiguous and sets up and nice sense of future competition between the the protagonists. Not too bad.