Enjoyed this much
more than Cold War. Unlike that episode, they actually took the time to establish a creepy atmosphere this time, and the professor and assistant characters came across much more like real people.
And I loved that the story veered off in such a strange, unexpected direction as well (if I didn't know better, I'd think Moffat actually wrote this, because that's more his kooky style). The time travel sequence, and Clara's reaction to it, was brilliant, and the stuff in the pocket universe was beautifully directed as well.
And while I understand those who'd prefer the monster be kept a monster, for me the revelation at the end was so nutty and inspired and off the wall that I couldn't help but love it anyway.