I suppose I was expecting something disappointing like the Silurian two-parter last series, notable only for Rory's death at the end. Instead we got something far darker, far creepier, and far more morally ambiguous in the good way.
The twists were predictable, by which I mean that Mr Pedia & I predicted them as we watched. They were still satisfying, mind you. We also had some far darker speculations that weren't borne out. One classic SF/horror version of this story, for instance, has all of the workers in the factory as Gangers all along, replicating the replicants while believing themselves to have been the originals. When the company sends the shuttle, it's to incinerate them all. But not on this TV show! Too dark.
Smaller points that struck me: How pleasant it is to see women as major characters on both sides of the conflict. We have a woman, Cleaves, whose defining trait is her tactical mind. We have another woman, Jennifer, who is an antagonist with motives we understand and agree with. The men are the disposable ones.
The pile of half-melted but aware Gangers was horrible. How did those people live every day with that in their presence? It nearly destroyed all my sympathy for them.
We never got the moment I wanted with Rory, where he makes the connection between his sympathy for the Jennifers and his own experience as an Auton.
Anagrams. Hmm. Can't make much of that.
The Doctor's deceit: a Trickster. Important to remember that.
Then we come to the reveal at the very end. Pond as a Ganger! The Doctor having suspected it for a little while! Rory trusting him and stepping aside. The Doctor sending the message to Amy that they're going to find her wherever she is. His anger as he cuts off the signal. Another classic horror trope with Amy, coming to awareness to find herself pregnant and in labor.
When was she taken? First guess would be while visiting the asylum during the Silence incident, but it's unclear. Why was she taken? "We do you honor." Who's the baby's father? It'll turn out to be Rory, because the show isn't that dark. Is the baby River Song?
The script offered us a solution to the puzzle of the season's first episode & the Doctor's death, then seemed to take it away. It might yet turn out to be related.