There are many things you should be careful about this book:

1. Don't let yourself deceived by the author's name. Just because she's the one who wrote about Harry Potter, it doesn't mean this book has the same atmosphere. It's a completely new deal. Therefore, don't be disappointed if "it's not like you thought it was", because honestly, the fact that Rowling can write about something this different and this real is the proof she's more than "the woman who invented everyone's favourite wizard". She's a writer, a novelist at her finest.
2. Some people, especially journalists who clearly pushed the fact that Rowling is the one who wrote Harry Potter, claimed this was "a modern comedy". It's not. A comedy by nowadays definition is something that makes you laugh and by classical definition is a story with the schema beginning-troubles-resolution-happy ending. This is NOT a comedy. Don't expect everything will end alright for the good guys and bad for the villains, and moreover while some characters are certainly villains in this book, they are not just "bad". They're first of all, people, and we all know only very few people can be defined exactly "good" or "bad", while the other 7 billiards are incredibly flawed. Don't forget that: they are not villains, they're just flawed people, like everybody else.

This said, I highly recommend this book to everyone that is mature enough to comprehend the reality. Have a good read.