I cannot recommend this book enough. I read it last December 2011 and it definitely was my favorite book of the year! I love zombie stories, and this book is one of my favorites (and the rest of the books after it as well!). After I started reading, I just couldn't put it down. I finished it in one sitting!
I also love the fact that the ones left sane enough are kids aged 14/15 below. I thought that was pretty original. When you're just a kid and every grown up turned into creepy flesh-eating zombies, what are you going to do? And I love how the book portrays that in such a situation, you don't only have these zombies to worry about, but the competition of resources as well. Since everything has turned upside down, the kids are left to fend for themselves, to the point that they have to fight each other in other to survive. I find this very much fascinating.
I could keep rereading this forever :)