"This Side of Paradise" is one of the top ten TOS episodes, IMHO. We get to see Spock let his hair down, so to speak, and experience human love and happiness, if only briefly. And the story has a strong but not heavy-handed message about idleness and the value of the Puritan work ethic. Interestingly, some viewers seem to dislike the episode for that very reason.
The scene where Kirk repeatedly insults Spock, trying to goad him into becoming angry enough to shake off the influence of the spores, is a classic. And the bittersweet parting exchange between Spock and Leila is beautifully written:
Leila: I love you. I said that six years ago, and I can't seem to stop repeating myself. On Earth, you couldn't give anything of yourself. You couldn't even put your arms around me. We couldn't have anything together there. We couldn't have anything together anyplace else. We're happy here. I can't lose you now, Mr. Spock. I can't.
Spock: I have a responsibility. . . to this ship . . . to that man on the bridge. I am what I am, Leila. And if there are self-made purgatories, then we all have to live in them. Mine can be no worse than someone else's.
Still brings a tear to the eye, doesn't it?