...is good. Read it. Spoiler alert.
(Not everyone loved it; for a comical review by someone who didn't get it and probably also reviews Chinese restaurants in terms of how Mexican they are, go here.)
Mieville likes designing monsters and also frequently defends science fiction as a literature of ideas. Both are on display here. Early on, before any real action, to keep things interesting, he inserts a monster that stowed away on a transport in hyperspace, something that must adapt to our universe and assemble itself out of matter and actions and relationships in this dimension. That was very cool. (I also think his cactus-men and especially mosquito women demi-humans in The Scar were pretty awesome too; not everyone gets as big a kick out of them as I do.) He also reimagines hyperspace as something called "the Immer", short for immersion; sort of the highest permanent reality, with ours only a pale reflection (characters mention our universe is the third one). One thing different about this take is that the Immer is really the basic, permanent universe and we're a temporary reflection built on top of it (usually hyperspace is conceived of as being "between" or "beyond" the real world).
It's hard not to think of Plato's world of forms - another connection to a Stephenson work, Anathem, where the world of forms is explcitly discussed as a literal real place - but I don't think that's what he was implying. He even solves the Fermi paradox by noting that the first human probes into the Immer picked up lots of communication and beacons: it's just that no one uses normal space. The most disappointing aspect of his flavor of hyperspace was that people get nauseous in the Immer. This is a heavily used convention in science fiction, but here I think Mieville falls for something that he avoids elsewhere: the idea that human (or any) biology would have a reaction and compensation for something it never encountered. For example, ionizing radiation is scary precisely becaues it doesn't hurt, or smell bad, or even irritate you, while it's killing you. No organism ever needed an alarm, because there wasn't any in our environment. Similarly no Earth organism has ever been in hyperspace/the Immer/whatever you want to call it, and a more interesting treatment might have been to have it damage us in odd ways that we aren't aware of until it's too late. Overall I'm pleased that he took an old convention and made it much more interesting; he seems to realize that you might as well because ultimately, whatever you call "hyperspace", it boils down to being a heaven or underworld that is described with sciencey-sounding language and that lets you get where you want to go in a reasonable amount of time.
I read this book mainly for the linguistic thought experiments, which were interesting, and reminiscent of Snowcrash and even moreso of Julian Jaynes's Origin of Consciousness in the Breakdown of the Bicameral Mind, a version of which the humans in the story directly induce. He's clearly thought through his ideas. I like it when an author explains a weak point that I hadn't identified; it shows s/he's doing his/her job. The explanations weren't always convincing but then again, it's fiction, not a linguistics textbook.
He also does a good job straddling the boundary of keeping the aliens alien, but allowing an interaction with humans that makes for good plot. I like how his here-and-there descriptive hints of the Hosts sort of gives you a picture, but not really: once he describes them as bird-insect-horse-coral creatures. He definitely does his best to avoid the intelligent gerbil effect. And he drops hints if you're paying attention that show there's a lot more to know about this universe's history: without saying so explicitly, several times we hear people mention Earth not as a myth, but as a place you can't get to anymore.
For my money right now Mieville's prose is the best in speculative fiction. My only complaint about him on this front was the over-dramatic single-line paragraphs in his Bas-Lag series, but those are gone. I hate having to still say this about science fiction, but his tone is very adult and nearly lit-fic. I wish everyone in the genre took their prose this seriously.
A sure sign that I really like an author is when I find myself making guesses about the person while I'm reading their fiction, or looking for little clues. For example, I know Mieville is a socialist so I can't help but look for allegorical clues or little lessons in his work, but I don't find any. He does have a few interesting comments about colonialism and racism in general, including one observation about the function of stories about natives going berserk and killing people when some minor social norm is violated (are those stories really about our insensitivity, or about natives being oversensitive and superstitious and stupid?) Not that his putting his political ideas in stories would be bad, even if you don't share them; I would mainly be wanting to see them done well, like Vinge does in Deepness in the Sky. What you do see in his work, quite clearly, are the messy unprincipled realities of politics. Normally I can't stand to waste my time reading about the politics of people and places that don't exist - I can barely stand to read about politics in the real world - but somehow Mieville pulls it off. I tried to figure out what he does differently and so far I haven't been able to.
Another pattern: in both The Scar and Embassytown, we see a female protagonist romantically involved with a morally very imperfect male who is maybe a little bit too sure of his own abilities and vision and value. This doesn't stick out quite as much as Paul Auster's odd insistence on killing his protagonists' families, but an odd amount of space is given to explicating the protagonists' lover's personality. Is this Mieville examining parts of himself he doesn't care for? A clever marketing trick to appeal to female readers? Pure speculation and maybe even coincidence, although again, Mieville does this well.
Probably my biggest disappointment with Embassytown was in the portions where the humans leave the city and go out into the Ariekene fields. We get some interesting ideas about Ariekene ecology and the mismatches that arise when alien evolutions collide (good) but very little of the sense experience of being out in alien fields. What color are things? What does it smell like? What does the land look like, and the soil and the rocks and the hills? (It's a good sign though that I care enough to be disappointed we don't get more of this.) I suspect the reason for his disinterest is that Mieville seems to be very much a metro guy, very much in love with London and the complexities of cities in general (that's quite clear in The Scar) and while he renders human landscapes expertly, he neglects natural ones. You see similar blind-spots in many American writers from New York, and (more often and more oddly) Los Angeles.