I like it. It does some clever, casual world building that has inspired later ST writers. Particularly introducing Surak and Kahless. They may not be accurate interpretations of those historical figures, but it opens up the thought process on the history of their respective cultures. And there's still one that we haven't learned more about in any novel or comics that I'm aware of: Zora. So even after all this time, there's still untapped inspiration to draw on.
The Excalbians are interesting, with their great power yet alarming naivete. It's been a while since I've seen it, but I remember liking the design of the rock-creature that interacts with the landing party. The flashing light in-time with it's speaking pattern maybe doesn't make sense, but I like the visual.
The episode has an interesting idea in terms of exploring which is going to be a more effective set of guiding principles to live by, good or evil. The episode doesn't delve into a more nuanced examination of it, but it makes the episode more interesting to reason that the Excalbians really are lost in terms of conceptualizing this dichotomy, they can't design an experiment very effectively, and (to my mind) they end up coming to a conclusion that is simplistic and wrong.
There's a lot of fun cat-and-mouse kind of stuff; so the episode doesn't stick with just fighting all the time. The deaths of the participants on Kirk and Spock's side I remember being effective, and a little disheartening.
I think it's a pretty decent episode. It's not a great one, but there really isn't anything wrong with an average episode that still manages to entertain.