I did find this one dragging quite considerably; while the overall pay-off was good, much of this could have been a lot better than it was. This said, full credit for some good use of time travel.
Doctor Who can be a very unpredictable beast at times. Just because one episode is a barn-stormer, doesn't mean that the next one will be - "The Caves of Androzani" was notably followed by "The Twin Dilemma"; BTW, I'm currently listening to the The Sixth Doctor - The Last Adventure from Big Finish and the Sixth Doctor is very well done there.
So, we start off with the Doctor discussing Beethoven and bootstrap paradoxes - both of which have turned up in the Whoniverse before. His liking for Beethoven was mentioned in the expanded universe in 2000 and the latter at at least three times in Steven Moffat stories; well... in fact you've kind of got it all the back to 2005. Then he plays electric guitar - Peter Capaldi's misspent youth coming up again. The fourth-wall breaking speech at the beginning was one of the highlights of the episode.
Clara continues to act as the Doctor's restraining influence; without her 'human touch', it's possible that the Doctor would have indeed actually gotten himself killed. She's definitely firmly ensconced herself in the role of the companion... which is going to make it even more of a pity once she goes from the show.
The base crew were good - I did feel sorry for O'Donnell when she died and the scene with Cass was very tense. Mind you, if you're going to murder someone, it's not a good idea to drag your axe on the floor, regardless of whether your target is deaf or not.
The Fisher King, while very well done in the special effects department, was otherwise uninspiring. He just seemed generic and not a massive threat in any way. He was disposed of rather easily I found and he is definitely not the sort of character that I'd want to return. I also was not keen on Prentiss, who I found annoying - as I believe I did the previous member of his race in "The God Complex" (I believe it was that one).
The effects in general were also very good - the flooding of the valley of course featured in the trailers, but kudos to the designers for the military training ground decked out to look like a Soviet town - these sort of facilities, suitably updated to reflect 'current' areas of operation i.e. the Middle East, still exist for the British Army.
The conclusion and the resolution to the mystery is a good one, although one does start to wonder if the sonic sunglasses are being overused. This said, I'm glad this story is over as it did feel a bit too long; not the best two-parter by a long chalk.
Maisie Williams is in the next episode; I hope this will be a stark improvement... get it, Stark improvement? I'll get me coat...