This episode has a few fairly good moments, but for the most part is a very average and rocky tale, and the closest the Original Series has come to a filler episode at this point.
The concept of Medusan ambassador Kollos, whose physical appearance drives people insane, is a thinly disguised reworking around the ancient Greek myth of Medusa, who would turn anyone who looked at her to stone. The idea here is an interesting one, but deserves a stronger storyline around it.
Probably the best thing about this mixed episode is Diana Muldaur as Dr. Miranda Jones. Muldaur had already played a different character in the second season episode "Return to Tomorrow", and in the role as Jones she has good screen presence. The twist revealed about her later in the episode (which I shan't give away here) is quite unexpected. (Diana Muldaur would, of course, go on to play Dr. Kate Pulaski in the second season in 'Star Trek: The Next Generation').
If this story had come earlier in the show's life, it no doubt would have been much sharper and polished, but by this point, things are feeling a bit tired, and the show has defiantly long peaked by this point.
The episode has its good moments, but ultimately winds up as one of the Original Series' far weaker stories. While there are some genuinely nice moments, there are also a number of weak ones.
The plot has a great many elements, and plays like one story after another. While it deserves credit for trying something with such scope, the result is sadly something uneven and unfocused.