All of Mieville’s imagination, put to such stunning and varied use in books like Perdido Street Station and Un Lun Dun, is here focused into one central conceit, into fleshing it out as far as possible. It helps that the conceit — the split and split-natured cities of the title — is a doozy. In the early parts of the novel, figuring out what is going on is the key; later, this switches to enjoyment as each aspect of the situation one hadn’t considered is brought up.
At the end, the book went where it had to. This is not a drawback.
Oh. And for a murder-mystery police procedural, the book is surprisingly non-violent.