I borrowed this off my nephew, who said it was "a little bit terrifying". I can see what he means, actually; I've read a lot of "Children's" books (quoted because not all children's books should only be read by children) and I've found the are generally two types: those that speak down to their audiences, and those that speak up to them. No doubt it is the latter that are the best, those that don't treat their readers like, well, children.
This book was, for lack of a better word, a little dark. The "terrifying" aspect would be the talk of murder and death, and the scary, horrible Burt who kills rats and turns them in to burgers.
From an adult perspective, it's a wonderful book, full of imagination and comedy, and some little references that I think only adults would understand. This is my first book by Walliams, whom I am a huge fan of in regards to his comedy, but it definitely won't be my last.