Frank Gorshin puts in a very strong performance. Okay, who doesn't? This episode, directly tackling racism in a rather heavyhanded way, retains its power even today.
If anything, the story gets biased when Lokai starts talking about building armies, with Kirk and Spock talking about how how Lokai himself is still alive while his followers are dead (and ignore Bele having done the same thing!), but otherwise...
Also, Spock again is mistreated. He can't discern the difference between the two Sharronians. Nobody sees it, but Spock should have. Yet he claims it's obvious both are duo-colored. Come on, Spock is hyper-detailed. He would not miss this obvious piece!! (Season 3 really puts Spock in a bad light at times...) But it's only for that scene where he's badly treated.
Of course, for being a low budget show, it's also a transparent ploy to make Bele's ship invisible. It's the one scene that's so badly cobbled together that it cannot be believed. Even Frank Gorshin couldn't pull it off, but at least he kept a straight face.
Definitely worth seeing for its powerful, provocative nature. But it takes some things (living 50000 years, shields, et al) a little too conveniently.