The science was REALLY wacky on this one, but I overall enjoyed quite a lot. I would have enjoyed more on first viewing had it not been hyped so much. Like robwebster, I think I should better check out from early spoiler free reviews and comments for the rest of the season, as those do more bad than good for me.
Anyway, a great story with some of the best acting we have seen this season and an overall defining situation for Clara as a character. Science aside (how did the moon increase mass, why did the shard disintegrate, how could a mass shift cause just one room to have zero-gravity effects and so on) it was really good and very refreshing.
Too bad the internet community seemed to hate it for one reason or another. I've seen this one labeled more than once as the worst episode in the show history, which to me, tolerant as I am of the other's ideas, seems very much laughable.