(Note that, as with all of my Original Series reviews, I am reviewing the original version; I have not seen the remastered versions at time of writing)
This ship-bound episode is very much an average instalment, and has touches of being a filler episode at times, but none-the-less has some interesting ideas which raise the episode up a few notches.
William Marshall gives a good performance as Dr. Richard Daystrom, the creator of the M-5. It is good to see a black actor get such a role in an era where it wasn't so common. The only thing I would say is that Daystrom needed a better, more distinguished costume – the one he wears makes him look like a lowly engineer or something.
As the M-5 starts to run amok, Daystrom continually defends and protects it, like an out of control child being protected by a parent refusing to see the truth.
Like many Original Series episodes, this story taps into the social status and fears of the time of which it was produced. The particular tale taps into the fears that computers and machines may one day replace humans in their jobs. It would be some 15+ years after this episode before computers really started developing and became powerful and complex enough that they could indeed be considered to replace a man's job, and in that respect the script is quite ahead of it's time.
The episode also seems to be a rather budget-saving outing, in keeping with it's slightly filler feel – the ore freighter that the M-5 destroys, for example, is recycled material of the S.S. Botany Bay from the first season episode "Space Seed". The resulting explosion is recycled from "The Changeling" earlier this season.
There are some dodgy effects as the M-5 computer takes aim at the other Starships – the other ships are all quite clearly models of the Enterprise, with four different shots awkwardly patched together. I understand that this has been rectified in the remastered version.
It is true that the 'outwitting a super-computer' plot has already been used in more than a couple of other episodes, and the story was verging on being overused. However here I did feel that it offered a different spin, with the Enterprise itself being controlled by the super-computer; it didn't feel overly like a recycled plot compared to some other examples.
At the end of the episode (as another reviewer has also commented), Kirk and co. see a bit over-cheery, and don't seem too upset that many lives have been lost, and one Starship, the Excalibur, has seemingly been totally destroyed. I would have preferred to see the episode finish on a more sombre, reflective note.
All-in-all, as I said at the beginning, a slightly average and filler episode, but with a fair story and some good ideas.
This episode does have special note to me, though. I have been a lifelong 'Star Trek' fan since birth, my father being a casual viewer which got me interested in the series at a very young age. But in the later 1990s and early 2000s, I did drift away from the series, not being so keen on some of the later spin-offs.
Then in around 2002/3, I was searching through some old VHS tapes I had recorded, and stumbled across an old recording of this episode that I had made. It was late at night and I sat and watched the whole thing; Even though it's not one of the best episodes, it pulled me in and reminded me why I liked the series so much. Since then, I have really got back into the series again (and even given some of the later spin-offs a second chance).