Surprisingly to me, many fans seem to loathe this episode. I saw it again this morning and it is still a series classic in my opinion.
There are definitely some continuity errors, and they never address the correlation between the planet being identical to Earth and third from its sun, the 'earth-style SOS, and what this cloned evolution has to do with anything, etc.
However, there is something eerily creepy about all these sinister kids hiding out and spying on/lying in wait for the adults. The scene with "mad Louise" who has just hit puberty still freaks me out when they show the close-up of her face. I also like the dialogue that goes on between Kirk, Spock, and McCoy in their frustration to find a cure. When Spock says in response to a slightly jealous Yeoman Rand, "[that girl] is at least 300 years older than you," I still crack up. The ultimate line is a frustrated Kirk shouting, "No more 'blah, blah, blah!' " Between that and his turning on that hideous, fake charm again for another lady (doesn't matter that this time, she's barely a teenager)…ah, just doesn't get any better!