Hooray for Moffat
I find that I don't have much to say about this episode, even though it was my favorite so far. I want to make one primary observation: What a difference the writer makes!!!! This is the first episode in season two that's by Steven Moffat, and he is galaxies beyond any of the other writers. His work is not always as funny as the others', and it's nearly always more profound. This was a very serious episode, continuing and deepening the heartbreak of the one before. My only gripe is that its plot gets repeated again later (in season five), but because of the cycling nature of the Doctor's adventures anyway, that's probably thematically intentional rather than evidence of any lack of creativity.
Before watching this episode, I was thinking, man, I hope we get an episode without silly aliens one of these days. There are no silly aliens in this one. There are clockwork men made by aliens, but they are more scary than aliens without being ridiculous. Nicely done.
I do have a question for the Doctor, though. When he messes up and arrives at the wrong time, too late or too early (or in the wrong place, for that matter), why doesn't he just go back in the TARDIS and proceed to the right place and time? I dunno. Any thoughts?