This is quite a phenomenal book, although it has some serious problems. It gets 5 stars from me for ideas, because one of the primary goals of good science fiction is to make you think and introduce new, fantastical concepts.
This book has that in spades. Mieville clearly has been reading some deeply philosophical literature on the nature of language and how we employ it, and has taken these ideas and made what-if scenarios with them. As in, what if someone turned you into a living simile or metaphor? What does that even mean? Normally this would be a topic for a linguistics class, but here, Mieville is considering these ideas, a few of many stunning concepts and questions, in the setting and trappings of science fiction.
One of the claims about science fiction is that the setting can be secondary to the story; Many sci-fi books would fit just as well in a medieval setting, replacing laser guns with magic wands.
In this case, the sci-fi setting is absolutely integral to us understanding the rational for the Ariekei and how alien they are to us. it allows us to really examine these ideas of language.
However, one it's very important to have a good story, a good yarn. In this area, Mieville's story isn't so sharp; much of the action was vague, and you'd read about someone's action, only to turn the page to discover everyone is at all-out war. The characterization was middling; I didn't care too much about Avice. Perhaps it's because Mieville focused so much on prose that he failed to make her real to me; everyone spoke in a fascinating way...just not a very believable one.
I like to compare this book to Vernor Vinge's A Fire Upon the Deep, which had big ideas too, namely the "empathic tethering" of the Tines, the dog-packs, as a big part of the story. That book explored what it means to be a "person" in an equally fascinating way. However, it was much better written and more accessible...and more exciting.
That being said, this book should win a Hugo for the ideas presented here alone. Fantastic read, and well worth checking out.