There is one word to describe this episode...empty. Its what the episode is, its how you feel when you leave it, its how the drama feels all the way through. The episode's primary problem is the lack of *anything* happening. It seems the whole episode is just an excuse to get a nightmare based plot out there to hammer home Atlantis's new mature vibe (seriously, the alien bursting from Teyla's stomach was unneccessary). This episode also dangles from the tree that is the season like a useless bit of fluff, with no real character development, or plot threads being laid down or tied up in this episode.
This episode also did nothing to help my opinion of Atlantis's new commander, Samantha Carter. She comes across as unbelievably pompous and the other characters seem to worship at her feet. In fact her arrival on the show sums up this episode in a microcosm, unoriginal. The writers couldn't be bothered to conjure a new character to take command so they pulled one out of the mothballs, much as they did with the plot. Now I like Carter, and Amanda Tapping's performance cannot be faulted, but Atlantis needs to be its own show, and this episode is testament to that.
The only real reasons for going out of your way to watch this episode is Joe Flanigan, who clearly relishes in his role as the nightmare Sheppard. The character isn't exactly threatening, but Flanigan manages to inject menace into an otherwise painty waisted villain. The other reason is, as always, McKay, specifically his dream sequence, which I won't spoil for anyone who hasn't seen it.
Summary, the episode is nothing special, I certainly won't watch it again, watch Reunion or Travellers instead.