Good episode. I liked it.
I liked how there was a bit more fear and tension than in most of the episodes in the newer series. I think Dr. Who has been missing that edge for quite a bit. There was also a degree of originality, which has also been missing over the last few seasons (just my opinion).
I think Matt Smith was rather good, although I wish he would get the jolly/ dark side of the Doctors character to be more evenly distributed. He is actually really good at being dark and serious, like Tennant was, and he needs to bring this out more often I feel (although that may be the script's fault admittedly...)
I know this probably won't be a popular opinion; but I really don't feel like Rory/ Amy work very well as companions overall. I don't think they really add anything, and they lack what companions are supposed to bring to the show - which is a genuine human reaction to all of these places. Amy still seems silly and childish, and although Karen is hot she doesn't add much to the show. She lacks any believable reactions; whether they are fear, melancholia or happiness. Not sure if that's intentional but the whole "stoic Scot" has been overdone if that is the case. I think Rory is a good addition; and brings nice contrast to the doctor, whilst being the most "real" character.
Compare Amy to Donna, Martha and even Rose and they were far better additions to the show overall in my opinion.
I don't have too many complaints about Smith; and I think most of his critics wrongly accuse him. I point the slight descent in the show's quality to be down to scriptwriting and the companions (whether that is also scriptwriting or just acting).
I still enjoy the show; but only because the recent series have lowered my expectations. But this episode was a step in the right direction.