It's somewhere betweena 3 and 4 for me. To be honest, the middle books in trilogies are seldom my favorites. They just tend to dredge up more problems and solve nothing, often ending in cliffhangers. 
 This one still has lots of action in it. Some new characters are introduced. Loyalties are tested and betrayed. I started feeling a bit numb about meeting new characters, though, since I was sure most would just be cannon fodder. 
 We're still following Thomas and his fellow Gladers, human lab rats dealing with misfortune and deadly situations at every turn, all through WICKED, a shadowy organization that we know next to nothing about. Indeed, I find myself unable to trust anything that is said or observed so I have no idea why anyone would do this. 
 I'm really hoping that the 3rd book can tie these things together - there are a lot of loose threads. I'm pulling for THomas and his firends still, and I'll be reading the next one.