The setting and language are the best. Setting: a huge grimy ancient city, its people splintered into a thousand subspecies castes. Interesting castes, too, the nearly-stone Balast, the anthro(?)bug Moths and Butterflies, Spindles and Deadheads and Blues and a zillion others. The Molemen are usury-butchers, who collect on unpaid loans by dissecting the valuable qualities out of their victims. The Adjuncs are chimeras stitched together out of dead flesh, used as police and shock troops. And above it all, the Rot: a black hole in the sky which once tried to eat the world, and will try again, soon. So that's creepy and eerie and horrible and wonderful, and entirely worth the price of admission.
The language is noteworthy. A number of important words in the setting are coined and work nicely: the Moth and Butterfly are 'Sectile' people. The butchers work in 'mogrified' flesh, and the Adjuncs are made of it. There are "gomorrah flies", and "ghasting suits", and stuff.
(One word notably does *not* work. Grist consistently uses 'mons' for 'mountain' or perhaps 'really big mountain', I guess after Olympus Mons. He frequently talks about "St. Ignifer's mons". But 'mons' does not mean that in English. It refers to a nice part of the female body. It's the Wrong Word and Grist *keeps* using it.)
Now the bad stuff. Bloated: The book really needs to be edited down by about half. There's lots of repeated material with slight variations, material repeated with slight variations, slightly-varied repetitions of the material, and repetitions of material, slightly-varied. E.g., the main characters have a lot of history together. We've watched every minute of it develop. They keep talking about it and more or less reiterating it. After the seventeen reminder of the time so-and-so injured sy-and-sy, it gets a bit tiresome.
Disappointing: SPOILER: the characters are trying to save the world, and they do everything right, and they still fail, and most of them die. I guess because it is book 1 of a series. Now, I *do* approve of having interesting twists in save-the-world plots. But this is a boring twist: the quests are performed, but the Rot is just stronger than the stuff that characters did, so it kills them all and nearly everyone else. (Of course by this time I was annoyed enough with them that I didn't mind a bit.) Still, it was a rather WTF sort of ending. It made the whole book seem futile, and reading it seem just as futile.
Well, anyways. It's a more interesting read than many. Three mysteriously-gathered child companions out of five.