This review is later than usual as I've been away in Vienna, Austria and was unable to watch the episode until today. I made sure that I avoided spoilers though.
As we heard towards the end of Steven Moffat's time as show runner, this episode frankly knocks it out of the park. As well as great acting all round – it took me a few minutes to spot one key twist – he's added a whole new level of horror to one of the show's most iconic 'monsters'; going to levels that were only hinted at when RTD handled them.
A brilliant episode from start to finish and next week's finale is something not to be missed.