I was surprised to see that this episode seems to split the vote somewhat, as in my opinion, it is classic 'Star Trek'. I really like the story, and it stands as one of my first season favourites.
In what is quite a rarity in the series, Captain Kirk takes somewhat of a backseat in the story, with his only real role being battling against Commissioner Ferris, who is pressing for the Enterprise to leave the scene to reach an important rendezvous.
The story belongs to Spock, and really shows up his 'logical' and at times angering and stubborn nature. I like the dynamics between the various passengers – and had two of the crewmen not conveniently (!) been killed by the creatures (reducing the weight for the Galileo to take off), I wondered if Spock would have chosen himself to stay behind.
Apparently, Yeoman Rand was originally supposed to be one of the members of the shuttlecraft, but after Grace Lee Whitney was dropped from the series, the character was adapted into another crewmember, Yeoman Mears. (By the way, I do agree with a couple of other reviews that she has very "girly lines" – "I bumped my head", "It's getting hot in here", etc.).
It is possible to pick small holes in the story, but I think that can be said with many 'Trek' episodes, especially in The Original Series.
In some ways, this is like a space-age version of the classic 'Flight of the Phoenix' (1965, based upon the 1964 novel). I find the episode exciting and dramatic in equal measure, and the climax really has you on the edge of your seat. All-in-all, an excellent episode.