[Dr.Jackson]

Enjoyed this episode, even though it did little to advance the story arc. It did give more back story to Sheppard, which didn't necessarily affect his character as it stands.
It was obvious that McKay was being affected by the machine when it started to go wrong. He kept leaving the hub of it, thus leaving himself open to the psychic attack.

I think that Teyla was not affected because of her unique part-Wraith physiology, possibly making her less susceptable to their psychic invasions. That and the fact that she's already gone probing the hive mind so would have built up any defences she had.