I can't quite decide on the rating, to be honest - wavering between three and four for a number of reasons, but probably closest to 3.75 stars, so rounding up.
The good: imaginative, fascinating plot; a writing style I thoroughly enjoyed; thrilling in a hard-to-put-this-down way at times; well-done shades of grey, with neither the German eugenics-enhanced superman protagonists being completely black nor the British doing-the-right-thing-at-any-cost-necessary protagonists being all white.
The bad: the characters felt rather cardboardy and wooden. Usually, this would be more of a problem for me, as I tend to be more invested in characters and care more for character-driven stories than for plot-driven stories, but this was an exception.
Definitely going to read the next book in the Milkwood Triptych, although probably not very soon.