This is an fine example of what I like about Stargate Atlantis. Nightmare are on all our minds and this gives us a glimpes what a person can influence.
And I really liked the way Sheppard and his evil counterpart fought and how McKay came to the rescue.
Nightmare can be scary, and with the influence of someone you care for are even greater.
I was sad to see that Heightmeyer was one of the victimes, but I guess someone had to die and sh was the most logical choise. She hasn't been used that much and her death put the edge on this episode.
An exellent episode, that I can watch over and over again