As a regeneration story, it works well. Unlike previous regeneration episodes, the Doctor knows from the start that he is dying, and the focus of the story is how he comes to terms with his impending regeneration(s).
There are some wonderful performances, particularly from Mark Gatiss as the Captain, as he realises the implications of the Doctor's passing reference to World War I. David Bradley is wonderful as the First Doctor, though the script perhaps overdoes the sexism a bit. Simply asking Bill to make some coffee would have been more in keeping than the lines about cleaning the TARDIS and the threat of a smacked bottom.
As a Christmas Special, I'm not so sure. Yes, it's snowy, and Christmas comes into it, in the form of the Christmas Truce, but the mood is scarcely festive. On one level the story is all about impending death: the Doctor's, the Captain's and those of the members of Testimony, even if it comes with a typical Steven Moffat 'no-one actually dies' sort of twist.
I'm not even sure about the nobody really being dead bit. When we last saw Clara, she might not have had a heartbeat, but she was off, zooming around the universe with Ashildr/Me, and Bill was similarly last seen flying off with Heather. If they are both part of Testimony now it seems rather more final.
All in all though, it's time for a change, and it will be fascinating to see what Chris Chibnall and Jodie Whittaker will bring to the series.
