Slow, but wholly robust, funny and mostly well-written. Very character driven. Good thing you got great actors such as Peter Capaldi and Maisie Williams to drive those characters then. There was very little here that felt unexpected in any way, though; this was the natural path they would take a character who's become immortal, and then try and draw parallells to The Doctor, which they of course did.
But it was well done, with the folks of the 17th Century (particularly Sam Swift) providing some good mood to an otherwise dim and strained atmosphere that surrounded The Doctor and Ashildr's relationship. As often when doing these character driven stories the underlying alien menace becomes a bit dull and boring, that's an unfortunate tradeoff. Overall I liked it a lot, but I have a feeling I'd be considerably less positive about it had it not've been for the superb acting.
Oh, but let me just say that I loved the ending scene with Clara. Bittersweet in a way; warm, but haunting and brooding at the same time. Very good move to have her away for an episode, it was refreshing, and because of that seeing her again was more impactful and made you happy (well, it made me happy, at least).