I had a mixed reaction to this book. On the one hand, I loved the whole magic system - the idea that an urban magician would use sodium street lights and electricity and rubbish in the same way that an old-fashioned wizard would use elements like earth and fire. The details about London were thorough, sometimes too much so. I found the use of first person didn't irritate me as much as it does in some books, but the MC's constant switching from 'I' to 'we' confused me. By about halfway through, I'd stopped noticing it too much - perhaps reading the previous books might have left me less confused over the reason! My main concern was the often long-winded and convoluted sentences in places which I occasionally had to reread. The kind of Neverwhere feel and theme to it was the original appeal, and one reason why I might read the other books from the series. 
 Update 11/10/15 I've now re-read this book in its proper place in the series. Makes much more sense if you read the previous two books first, lol (would help if the books had been clearly marked as which book in the series they were from the start). Still my favourite cover of the three.