I had just read this book last week, and thought it might be a good time to see the movie. It's got a great cast, a great director and while the book often seemed somewhat unfilmable when I was reading it, surely they would have done a good job when it came time to adapt it. Well, not a good job, but it could be worse.
They changed massive parts of the book, and many of the changes made sense. They needed to eliminate characters and streamline the plot, and I'm fine with that. I was disappointed that they killed off (and this is not spoiler, as it is stated very early in the film) both of Jodie Foster's character's parents. The relationship that character has with her mother and stepfather is so important to understanding her as a person in the book, and with that missing, I felt that it hurt the character. They made some other changes with her character that I also felt made her a weaker character, particularly in her relationship with Matthew McConaughey's character. Honestly, I felt this main female character was one of the best parts of the book, and I was very sad to see her in this watered down version.
That all being said, the other thing that's great about the book is carried through to the movie (not quite as effectively or forcefully in the movie, but it's there) and that's the debate between science and religion. The movie raises some very interesting questions in this regard and I'm willing to cut it a little slack for that reason. Most movies don't deal with religion, unless they're made by a religious group, and then there's no debate on any questions, just blatant proselytizing. I appreciate this movie at least trying to examine religion in a more intelligent way.