The premise is that of a far future interplanetary empire. Humans have colonized hundreds if not thousands of planets, and there are huge ships and space stations with superhuman artificial intelligences. These AIs can use human bodies as "ancillaries," avatars of sorts. The main character is one of these ancillaries separated from their ship's AI. By now the parallels to Bank's Culture are probably apparent.
One particular, if not entirely believable, aspect is the way the book deals with gender from the viewpoint of a genderless language and a genderless machine. It'll cause any future Finnish translators a lot of grey hairs. While the rationale behind introducing the verbal trickery may be a bit suspect, the effect was pretty interesting, at least to me.
The language and prose were well crafted, and it's clear time was spent on this book. The pacing is a bit slow — this isn't as much space action as it is a more measured "what if" exploration, which speeds up towards the end. Unfortunately I felt like many of the moral issues were not entirely satisfactorily dealt with. Since the main character isn't human, criticizing them for not really getting all the depth one might want seems churlish.
Overall, I'm a bit torn; if I look at any single technical aspect, Ancillary Justice is a good enough, but not great. And yet it kept me turning pages and neglecting chores and sleep; it clearly is better than the sum of its parts, and all of those parts are perfectly serviceable, if not excellent or particularly original.
Three and a half stars, and I'll definitely pick up the sequel. The plot is self-contained; things are fairly well wrapped up making the read rewarding on its own, but there's a clear larger story arch that has been set up. At least in the Kindle edition, after all the marketing fluff, there's an interesting interview with the author. (And thank goodness, this book knows where it ends, rather than the Kindle thinking I have to page through all the previews for the book to be finished.)