Great idea to kick off with, but it never reached its potential.
The evil Kirk was the more interesting half, and the episode analyzed his traits fairly thoroughly. On the other hand, it seemed to imply that the good Kirk was handicapped, but I was never satisfied that an answer was clearly indicated for why
that was the case, beyond the general weakening that both halves experienced. Of course, by the title, it advertised itself as about the evil side, instead of both sides equally, and it delivered on that account.
I don't understand in what century Spock's rhetorical question at the end was appropriate.
The whole landing party in peril was a pretty big plot hole for lots of reasons. According to Memory Alpha
The subplot of Sulu and three other crewmembers stranded on the planet were not present in Matheson's original script, and was added in staff re-writes. Matheson did not like the idea, as he explained, "I hate B-stories. They truly slow the story down. (...) My script stayed entirely with Bill [Shatner] having this trouble of his two selves, on the ship. (...) They added a whole subplot about people down on the planet, ready to freeze to death, because they have [a] transporter functioning problem. (...) I stuck entirely with Bill."