I was pleasantly surprised by this one. 
I picked up the audiobook and was surprised at the length, and I also went into it not knowing anything about it.

The beginning was quite slow and I couldn't really see where things were going, but then after Billy went to his Dad's it really picked up and did so again once the main crux of the story (the greys) got into it. 

I thought this was done really well - it handled Billy's mum's paranoia, the strained relationship between Billy and his Dad and new step family. The new friendships. It's all just done so well and it feels like a really good foray into dystopia for kids. It was exactly what I needed right now.