Probably a 3.5. I have been reading this series on my iPhone, because the style suits the medium. I can read a few minutes in line or at the dentist without losing track, and it dilutes the overwrought tone that sometimes takes over these stories.
I did guess the twist almost immediately but it was sufficiently interesting watching Will figure it out that I wasn't annoyed by how long it took him. I was a bit annoyed by how clingy and pathetic he could be, but he did grow during the course of the story. Heath was moody and unreliable, but for very good reason. The background characters were quirky and fun.