When Mieville is on, he's on.
All his books have parts where his writing really shines, but he doesn't seem to maintain that level for the entire book. They all seem to have parts that are a bit of slog, frankly.
City and the City, in my opinion, is the best example of him maintaining a high level of writing from cover to cover. Perhaps
Embassytown and
Railsea are in that club as well, but I've not gotten to those yet.
Take that from a reader who still considers trudging through Mieville's sloggy bits to collect the shiny bits, well worth the effort most times.