You don't hear this episode mentioned much, but it's a curiously good episode, given the show was basically shutting down and the third season is generally considered as a downhill slide. What could be a schlocky jump-the-shark concept - "Enterprise Meets Lincoln!" turns out to be a curiously thoughtful exploration of good and evil.
It's also clear the episode struck a chord since later writers would use Kahless, Green, and Surak first seen here. In a way this episode forms a trilogy with "Amok Time" and "Journey to Babel," showing us Vulcan society and mores. Barry Atwater, a talented second-string actor, portrays the Father of All Vulcans with suitable aplomb.
Lee Bergere, another veteran actor, gives a deep thoughtful performance as a Lincoln who isn't heroic, but clearly a man who has been torn by leading a country through a bloody civil war.
On top of these two performances there's the deeper significance that the two characters arent' real, but are manifestations of Spock's and Kirk's imaginations of what these characters would be like. Look deep enough and you can see what it says about those two characters. So it's much more then simply "Lincoln beams on board the ship."
Philip Pine gives a suitably slimy performance as Colonel Green, although the other three "evil-doers" get short shrift.
The other interesting aspect is the Excalbians, who comes across as a totally alien race, not in appearance but in concept and intellect. The idea of pitting good vs. evil doesn't seem to make much sense, but then again probably a few of our concepts of resolving disputes and settling intellectual issues wouldn't make sense to an alien, either. The concept is graspable - "if good is stronger, it should win in a fight" without seeming totally stupid, and seems like something an alien race might come up with.
We also get a McCoy and Scotty who are still feisty and anything but yes-men, and a few comments on race and advancement in the future. And lots of quotable lines (see Quotes).
Overall, a surprisingly pleasing episode, and one that caught on enough to act as a springboard for several future episodes in other Trek series.