I struggled with rating this.
On the one hand, I really didn't enjoy it very much. As in, every time I picked it up to read it I had the urge to go and do something else.
On the other hand, I think this is more of an issue with my thoughts on epic fantasy than with the book.
Because, let's be clear, The Mirror Empire is not a lazy work of epic fantasy. By which I mean: Hurley takes the trouble to imagine societies, worlds, which are culturally very different from our own, as opposed to recycling sanitised medieval societies in which women are more or less optional (*cough*Magician*cough*). The first in a trilogy (I think?), The Mirror Empire is set in a world where plants are carnivorous, and where magic-users (I like that Hurley uses the word "gifted" for these - because if you grew up with magic you wouldn't think of it as "magic") are dependent on the movement of the stars for the strength of their power. The dark star Oma is rising, meaning that hundreds of parallel worlds are coming together, which has always signalled cataclysm in the past. Three enemy countries (Dorinah, Saiduan and Dhai) with very different social mores find themselves having to cooperate (sort of) to save their own world.
Very different social mores, did I say? Yes - and this is where Hurley is particularly awesome. In Dorinah, women are the default just as men are the default in our world (and this isn't necessarily a good thing, note - Dorinah's women enslave and sexually abuse "their" men); in Saiduan, the language allows for three genders, male, female and ataisa; in Dhai, polygamy is normal, with adults of either gender commonly marrying into already polygamous family units, and having lovers as well. In both Saiduan and Dhai, gender is separate from biological sex, and the Dhai can choose their own gender identity. There's a Saiduan character who feels that they don't fit into any of the three gender options, "neat little boxes" as they describe them. Hurley's work with gender and sexuality, and the various power relations involved (I'm thinking also of God's War, which I loved) is spot-on and much-needed and if only The Mirror Empire were a different story.
I think what the issue was, for me, that there was simply too much going on. Hurley doesn't do infodump, which is great, but it's also exhausting if you're going into a 500-page novel covering three countries, about a million different characters, an unfamiliar magic system and a highly complex and involved political situation.
I also thought Hurley's prose was (I'm sorry) turgid: it was an effort to wade through, and I found some phrasing occasionally clumsy.
Which all, essentially, boils down to: I just don't think epic fantasy is my bag, and that's OK. But, if you are an epic fantasy reader, definitely, definitely, read this, just for the gender politics.