I almost never say this, but I really liked the movie much better.
For the first half of the book--basically until Will meets Rachael--the book and the movie are virtually identical. I almost just gave up on the book in boredom, because, to paraphrase a friend, watching Hugh Grant = so much better. However, I persevered.
In the movie, Marcus sings at the school concert to try to make his suicidal mum happy, even though it will be social suicide for him. Will shows up at the concert at the last minute to play back-up, thereby deflecting Marcus's humiliation onto himself. At the end, you get the idea that--although the characters are still quirky--they'll all be OK because they've formed this little family.
In the book, the concert scene is replaced by Marcus taking a trip to see his father with Ellie. They stop midway, and get arrested when Ellie breaks a window. Everyone--Will, Fiona, Ellie's mom--comes together to go get them, and that's kind of it. The book ends, not with the idea that they'll all be OK because they've got each other, but rather suggesting that Marcus and Fiona will be alright because they've assimilated and become "normal".
The book might have been more realistic, but it was disappointing. Just watch Hugh Grant, trust me, you'll like it better.