I am not sure what to make of this book or its author.
Considerable time is spent attacking cliches of the fantasy genre, yet Mieville's Unlondon is acknowledged as derivative of Gaiman's Neverwhere. Isn't being obviously derivative of your predecessors both a cliche and the worst crime of the fantasy genre? (I give credit for open admission of the debt, though.)

One of the cliches attacked is that of the the Protagonist with a Heroic Destiny (PHD). Fairy Nuff, but you can see the attack coming from about page 5 and what form it is going to take: a Protagonist Switch (PS). Now for me, at least, the PS is a far worse literary crime than dragging out the PHD one more time. An author can only get away with a PS in a Greek Tragedy (or likeness there-of e.g. Shakespeare's Julius Caesar). Un Lun Dun is not a Greek Tragedy - the Initial Protagonist doesn't die horribly.

There are all sorts of daft elements in the book, such as the Unbrellas that are created when umbrellas are broken or the pet milk carton so it would be tempting to catagorise the book as a spoof on the fantasy genre - but spoofs at least try to be continually funny. Un Lun Dun does not; there are funny incidents and funny puns (Binjas?!) but the humour is sporadic - so this is not Terry Pratchett's Disc World, either. Much of the time it wants to be a straightforward fantasy adventure. Eoin Colfer and Partick Landy have successfully created imaginative, humourous fantasies that are also gripping - but they have a uniform tone - this does not. It's not really comic relief, either - it's just random switches of mood.

It's difficult to accept an author tacitly criticising his peers' use of cliche when he can't manage proper plot construction and use of tone to match situation and intensify reader involvement. Mieville has tremendous imaginative invention but he is not in control of his art.