This is a very good first season episode, and it is interesting to see Captain Kirk "split in two", with one half good, one half bad. Although limited by the effects of the time, there is some good lighting and the odd clever jump edit that brings the "bad" Kirk to life.
I have always loved 'evil twin' scenarios, and the second season's classic "Mirror, Mirror" has always been one of my favourite 'Star Trek' episodes. Likewise, this episode taps into a similar vein, and it is great to see the good Kirk, unable to make decisions after being split in half, try to get his bad half under control.
There is a noticeable mistake running through the earlier parts of the episode, where both Kirk and the bad Kirk have their insignia missing from their uniform tops. Apparently it had been removed when the top was dry-cleaned, and nobody noticed that it hadn't been reattached – pretty slack on somebody's behalf, and surely someone must have noticed! Further bloopers appear later in the story when the bad Kirk is on the bridge – the viewscreen behind him can be seen to be blank; and the scratches on his face jump sides on close-ups (due to the film being reversed).
Anyway, the story is a good one, and the added twist of Mr. Sulu and co. being stuck down on the freezing planet, unable to beam up, adds some extra urgency to proceedings. It did strike me, though, why a shuttle wasn't launched to pick them up – even if it would take time getting there, it surely would at least have been a back-up plan. Or maybe I missed something there.
As good as the story is, if I was forced to be critical, I would say that it loses its way in the latter stages. After the bad Kirk has been captured, and the good Kirk tries to bond with him, the story seems to go slightly limp, and lose some of its urgency. Likewise, the bad Kirk doesn't actually seem all that bad later on – he just shouts and rants a lot. But these are more just nitpicks than anything. All-in-all, a good story.