Was the 'evil twin' ever done before this?
It seems to have become the progentior of the genre, anyway. Most TV shows that have done the idea since seem to have taken some sort of inspiration from this.
It's arguably lost some of its impact because of this.
Evil Twin plots having become a cliche.
But speaking personally, I still find the setup satisfying.
I really like the idea that it isn't simply a case of "Good Kirk" and "Evil Kirk" -- in fact, both sides of his personality are a part of the sum total that is James T. Kirk, and "Good Kirk" needs his darker characteristics in order to be an effective leader. The so called 'evil' Jim Kirk has got all of Kirk's confidence. One side can not live without the other.