The descriptive writing is over-the-top; the narrative writing even more so, dipping at whim into hallucination or stream-of-consciousness or madness-of-crowds. More, Mieville is pushing hard against genre tropes. His protagonist is cold, disdainful of social contact and emotional display. She does not fall in love or discover her true calling in this story. Her environment, the entire world of Bas-Lag, is described in terms of ugliness, cruelty, sickness, sin.
But. Nonetheless. The story grips you and hauls you away -- it did me, I'll say that much. Bellis Coldwine is on the run from New Crobuzon (she will eventually say why). She figures a few years on another continent will settle things down. Instead, her journey is hijacked by Armada, a floating pirate city of lashed-together ships and social customs. And then -- plans, plots, schemes open and covert, lies, betrayal, boom-boom-boom until the last page.
There is just enough warmth -- conditional and hesitant as it is -- running through the story that it is a story, and not just a gratuitous assault on my reading-senses. There is just enough titanic wonder under the broken landscape, just enough beauty. (Again, for me. I can easily imagine that this is an individual balance.)
If that were all of The Scar, I would have been happy with it. It is more. As I wrote of Embassytown, Mieville has a gimmick and he makes it work.
The Scar is ultimately about Armada's quest (or the quest of some Armadans) for the Scar, a vast distant crack in the world. The Scar is a fracture of physical law, of reality, of certainty; the Scar was caused by forces that broke Bas-Lag long ago. Implicitly, the Scar is the cause of all the transgressive impossibilities of Mieville's setting. It is the First Cause of his fantasy world.
Armada never exactly reaches the Scar. That's the point. They turn around at the last minute and head home -- sort of; in a sense they don't. It is frustrating, a contravention of the accords of epic fantasy, and that's the point -- everything breaks down at the Scar, up to and including your quest plot and the resolution of what happened. Mieville fantasy world is broken. Live with it.