Ok, first thing:
I saw the movie befoee I read the book. In terms of gay cinema, it's a really cute movie. Considering the slew of badly overacted or badly written/produced gay cinema out there, getting one that's really cute/great/well-written can be tough. Now, having noticed that the novelization of the movie is based on the screenplay, I'm going to say that I have more respect for the actor who plays Aaron, I really feel like the material that he was given had very little depth. At times, the novel felt sluggish, and there was no real dimension to the emotional upheaval that's supposed to be happening.
In addition, (and before anyone throws something, it may just be me,) Christian has no depth. One of my first reactions after seeing the movie was that the Christian character was a complete miscast. Although there may have been chemistry between the actors themselves, the actor's range falls flat for the movie.
As a rule, that's generally my usual complaint about gay cinema. If the story's great, the acting's awful. If the acting's awful, the story's so cliche that you might want to stop watching. With the notable exception of a few amazing Hollywood contributions, (Brokeback Mountain, the Hours, Milk, etc.,)and several fan-favorite indie pieces (The Sum of Us, Beautiful Thing, Priscilla, Queen of the Desert, etc.) most of the gay cinema I've seen is so full of cliches and over/underacting. Here endeth the rant.
Getting back to the review...for me, the movie's cute, the novel falls short. There's a range of emotion and depth in the movie that just doesn't cross over into the novel.