Ancillary Justice is a powerful experience to read. One of the most immersive parts is the book’s approach to gender and worldbuilding in general. All Radchaai citizens go by she/her pronouns and gender effectively does not exist (leading to some conundrums when Breq, on other planets, needs to appear non-Radchaai and has to figure out the concept of gender on the fly). As I read and found myself inadvertently gendering characters, this part of the book led to some interesting self-reflection. The worldbuilding around the concept of ancillaries are also fascinating: the Radchaai conquest formerly had an aim of collecting humans in suspended animation to later be ancillary bodies, with implants forced into them that effectively killed them and slaved their bodies to the AIs of ships; however, new reforms in the Radch have banned “manufacture” of ancillaries although ships continue to operate with ancillaries, which they consider parts of themselves and near-impossible to live without. The prose is also very sparse with just enough description to let you imagine a whole world outside the characters—think J.K. Rowling.
Ancillary Justice is highly acclaimed, but the plot itself is not the most compelling independently (the two sequels, Ancillary Sword and Ancillary Mercy, fill in plot holes). I was compelled enough by the concept of ancillaries, Radchaai culture, and Breq’s character to keep reading, but if you can’t immerse yourself, the plot can seem to drag. If you can stick through it, though, the story picks up in the latter half/third and definitely gets better in the later books.
It’s not for everyone, but I definitely liked reading it and couldn’t put it down—I’m still rereading! Something about it struck a chord with me, though I can't put my finger on it. It's immersive and gorgeous, a space opera in every sense.