China Miéville is obviously a man who loves words. Reading his long, often complex sentences, you can tell that he chooses his words very carefully. He also has an immense imagination. Both were put to very good use in Perdido Street Station.
The plot itself is absorbing, once it kicks in. It takes awhile for that to happen in earnest, but every page before that builds up the stage for the main drama of the slake-moths. Pay attention, everything is important. As a knitter, I take a very dim view of moths in general (they are, after all, wool's natural predator) so I could certainly buy them as psychic predators, and they were indeed frightening. But that's only part of what the book is about.
Perdido Street Station is about the city of New Crobuzon, vivid and grimy and utterly fantastic. It isn't the sort of fantasy world I'd like to live in, but it is endlessly fascinating to read about. And yet, with all the fantastic elements China Miéville included, I could tell that he'd barely scratched the filthy surface of New Crobuzon.