One of the best and most memorable episodes of the original Trek series.
First off, the "star" of the show: the gigantic space-amoeba critter was such an impressive sight in the original series, that when it was re-released in the recent "enhanced" episodes with remastered HD format and state-of-the-art effects, the creature itself was left virtually unchanged. In fact, IMO, the critter looked better unenhanced.
This is doubly impressive in light of the show's budget limitations, being a relatively inexpensive process put to very good effect.
On top of that, the writing doesn't fall into the bogus-science 'treknology' gibberish and continuity problems that plagued and dragged so many episodes into mediocrity. The science in this episode is rock-solid, one of the few Trek episodes that qualifies as true hard-core science fiction.
The story itself is slowish for Star Trek, more brooding than action, but it works without insulting your intelligence in the process.