At the beginning of the book Mieville thanks Kafka and Chandler and that is entirely apropos. How you feel about The City & The City will depend on how you feel about that mixture. The basic premise is that there are two separate that lie alongside each other. The two cities are separate places that share the same location in time and space. The two cities are kept separate by their inhabitants ignoring the other city. Mixed with that Borges like premise is a murder mystery told in the first person in a gritty style. The effect is as if you set the Maltese falcon in Borges Library of Babel.
For me, that does not quite work because the metaphysical fantasy pulls in a different direction than the realism, so that it becomes hard to suspend disbelief and one does not know where to suspend ones disbelief. Also Mieville's prose is very standard best seller prose, like Crichton or King or someone like that. I also found that the end solution to the mysteries was less interesting than the red herrings. Also, I think that more could be done with the premise. It might be used as social commentary or an analogy for something, but not much is done with it. On the other hand the idea is really interesting and clever. Mieville is a clever writer, but He needs more poetry in his prose, or he needs more ideas per 100 pages, or he should write short stories. There is a real mixed bag here with a lot to admire. Nothing is bad, but there is a lot that is fine, but not memorable.