I have only read two of his novels..the aforementioned Perdido Street Station and The Scar. I view both of them as two of the better novels I have ever read in my life and look forward to sampling more of his work in the future.
Personally, I find his characters to be extremely likable. In Perdido, the relationship between Isaac and Yagharek is something I can recall to mind with relative ease, despite reading the book a few years ago now. In my old age of 33, that is pretty impressive. In particular,
the ending was extremely impactful to me due to the relationship they had built with one another throughout the entirety of the novel. When Isaac truly understood, at the end, the impact of Yag's crimes, it made the choice in front of him truly heartbreaking.
Despite my enjoyment of the character in his books (The Scar
fits the same mold for me as well), I do agree that it isn't necessarily the selling point to a Mieville novel. I feel like his world building ranks him at the top with few peers to challenge him. My reading is still limited though, both in the genre and for his works, so perhaps that will change. But from the two I have read, I feel he is one the best authors I have ever read, regardless of genre.