My first introduction to Miéville's writing was Railsea. I had an almost instant reaction to it (confusion, frustration, amazement) that only grew stronger as the story progressed (and changed into shock, awe and adoration, and finished with the literary equivalent of being hit on the head with a shovel). Un Lun Dun disappointed me for the supremely unfair (on my behalf) reason that it didn't draw much of a reaction from me.
I think that where YA and children's books are concerned, there are two essential kinds: those that cater only to the intended audience, and those that transcend these limits. Railsea is the later, Un Lun Dun the former and therein lies my disappointment. Of course, there is a third category - those that are so terrible that NO ONE should like them, but that's totally irrelevant in this case, trust me.
The story is vividly imagined, totally nuts, colourful, bright, simple to read and has strong characters and a good message. Perfect for a children's book. Miéville cites Neil Gaiman as a huge influence, and this influence can be seen throughout - if Gaiman had written Neverwhere for a young audience, it would be something like Un Lun Dun. This is not to say that there is any blatant copying, or that the writing style is the same, only that the inspiration is plain to see. There is nothing wrong with this book. I just somehow wish there was a little more to it that would broaden its appeal to an older audience. This is no fault of the book's though, and who am I to criticise the author for simply catering to the intended audience? In fact, this book is now on my Christmas gift list for the younger siblings/cousins, who I know for a fact will absolutely adore this.