Excellent science fiction. The plot is difficult to explain--check out this review at Dear Author, which convinced me to actually buy the book, to get an idea. There's a lot going on in Ancillary Justice and a brief review can only shortchange it.
That being said, I'll make two points. A lot of discussion around the book focuses on the fact that the protagonist, Breq, has no gender and has a very difficult time recognizing gender in others. 'Gender' is not a natural concept to Breq and Ancillary Justice is told from Breq's POV.
The result is not always fluid. It is not a seamless production. But when I finished reading Ancillary Justice and moved on to another book, I was honestly shocked and dismayed by the emphasis on gender in it. Every gendered adjective and assumption leaped out at me as painful and wrong.
So I would call that a success.
But here's the other point. A lot of the chatter that I've heard about Ancillary Justice revolves around the high-concept sci-fi elements. But the thing that moved me most about the book, the thing that carried me through it, was the character of Breq.
At first, Breq seems to either have no emotions, or to have repressed all emotion until the result is nearly the same. But as the book goes on, weaving together flashbacks and the present, a deep well of--actually, I want to extend this metaphor. First it seems like a deep well of feeling, narrow but profound, but then we dig deeper to the whole aquifer, rivers and currents that extend in all directions and will never run dry. It's amazing.
I loved Breq at the beginning of the book, as a stone-faced badass. I loved Breq even more at the end, when I knew what that facade hid.