It is often said that the third season is the weakest of the Original Series' three seasons, and I agree with that. But that said, here we get another enjoyable episode, the third in a row (following "The Tholian Web" and "Plato's Stepchildren", both of which I also really liked).
Some don't like this episode, and indeed there are some major nitpicks and inconsistencies. But in fairness, that can be said for a huge chunk of 'Trek' episodes if you look closely enough; I find it better to put those niggles aside and enjoy this intriguing story.
The story of 'high speed' aliens is a good one; probably one of the third season's most interesting ideas, in my opinion. At first you are left to wonder what the high pitched 'buzzing' is, and the explanation unfolds well.
Deela is well played by Kathie Browne, adding another 'Star Trek' beauty to the ranks. Her character and Kirk's work well together, and I felt there really was a spark between them.
Along with the first season's "Operation –- Annihilate!", this is one of my very first memories of watching 'Star Trek', or indeed any television in general. I was about three or four years old, and I can clearly remember the aliens moving at high speed, sounding like flies, and Mr. Spock working out to slow down Kirk's message.
(It is possible that "Operation -– Annihilate!" and this episode were shown close together, as the BBC's run of the series in the 1970s/1980s often jumped around the seasons and followed little pattern).
All-in-all, although this isn't a commonly popular episode, I personally like it enough to give it a very reasonable 9.5 rating.