That was really good. Definitely one of the best episodes of the last few years.
I love that Ford took the Dalek story in a different direction, with a lot of the episode being set inside the Dalek and not even overtly involving them. Dalek's having existential crises have been done before (such as in Jubilee and Dalek), but this was different enough to still feel fresh. The Dalek being inspired by seeing how life continued to spring up in spite of everything they did was a really interesting concept, and I liked how the episode addressed the prejudices of the Doctor firsthand, and actually acknowledged some of the problems with how the Daleks have been treated by him over the years. It was great to see Clara call the Doctor out (this was one of the first times I've really liked her, as opposed to just tolerating her), and to see the Doctor actually getting his hopes up about being able to redeem a Dalek (and possibly more) in some way.
This made the climax of the episode really strong emotionally, from both the Doctor and the Dalek point of view. You can feel how he is crushed when the Dalek starts talking about hate. It's a callback to Dalek, but it works well, and it shows both the problems with trying to redeem a Dalek as well as the possibilities which do exist. Capaldi is on top form throughout the whole thing, too. He really sells the darker, more manipulative Doctor ala McCoy very well, especially early in the episode when he allows the soldier to die in order to follow the trace of what happens to him, but he also shows his compassionate side as well. I think he's handling the idea of a darker Doctor very well.
From an imaginative point of view (having the climax of the story being the Doctor trying to broaden the view of a character by opening up their consciousness and showing them things they could never imagine), an atmospheric point of view (just about every scene inside the Dalek) and from a dramatic point of view, this is very solid stuff. It's not an all time classic, but it's comfortably one of the better episodes of the entire Moffat era. I haven't really felt this good about an episode since A Town Called Mercy.
Ford shows his skills when it comes to fleshing out the supporting cast, as well as peppering in some very snappy dialogue. He also goes back to the reverse TARDIS introduction scene which he used in The Waters of Mars as well, but you're allowed to repeat yourself at least once. Even Murray Gold's music at least stays out of the way for most of the episode, even if it isn't particularly great. The only real issue I have is when Gretchen's death scene is immediately followed up by the bit with Missy. It was poorly edited and unnecessary, and it sucked out some of the drama from her sacrifice.
Hopefully this is not the last we hear from Phil Ford on the show. He's two for two now, and he was always a reliable writer on The Sarah Jane Adventures. This is what Doctor Who needs more of.