I give the episode, and the entire season with Capaldi, a resounding "Meh."
I want to like this new Doctor. I want a dark, dangerous and potentially Chaotic Neutral Doctor (instead of the Chaotic Good we've had since it returned). I like Capaldi as the Doctor, in theory. But... there's just something missing.
Perhaps it's because I was hoping for Danny Pink to become a new Companion for the Doctor, and got my hopes up too high. (I think I'm one of the very, very few people who actually like Danny Pink. Not merely tolerate him, but I actually like the character.) Perhaps Moffat's hand-waving of basic science (see: Kill the Moon) in favour of... well, sub-par stories full of huge leaps of logic that don't really have much of a place in a series like Doctor Who has reached a point where I'm not willing to forgive the leaps of logic anymore.
The last year or so of Matt Smith's run I felt the same way; I thought it was just the let down of finally getting rid of Amy and Rory (two characters who had grown stale in my eyes) only to find that Clara wasn't nearly as interesting as a permanent companion as she was a recurring companion. But the resounding Meh continued on to the specials from last year and all of this season. So I'm thinking that what is making it Meh for me is Moffat.
I think it's time for him to step back and let someone else take over.