I wasn't expecting much from this episode, but it turned out to be a very good instalment.
In many respects, this story is rather like the first season's "The Corbomite Maneuver", and "The Doomsday Machine" earlier this season, in that instead of a new planet to beam down to, and / or a new space creature to tackle, instead the Enterprise is threatened by a giant space entity.
The episode is captivating as the amoeba just seems too vast and too powerful for the Enterprise to escape. Of course, we all know they'll survive in the end, but HOW...?
Kirk being forced to choose between McCoy or Spock going on the shuttlecraft mission into the amoeba is great, and one of the best sequences of the second season in my opinion. Both the writing and acting of it are of a very high quality.
The moments when Spock is assumed to be dead as a result of his exploration are also of a high standard, and play out excellently.
Then there is one of my favourite pieces of dialogue – McCoy "Shut up Spock! We're rescuing you!", to which Spock replies "Why thank you, CAPTAIN McCoy".
On hindsight, this may actually be one of the best episodes of the second season. It's certainly amongst the best down the latter end of the season.