I always find that books are better if read before you see the film. You use your own imagination rather than the images from the film and you get the story as the original author wrote it rather than someones interpretation of it.
Yesterday I wanted to watch 'The Boy in the Striped Pyjamas' because it had come from Lovefilm quicker than I expected it to. So, I started reading the book when I woke up, finished it by lunch time and then watched the film in the evening.
However, I think it was not a good idea to do both so quickly, because inevitably the film had changed a lot of the detail, and as the detail was still so fresh in my mind I noticed all the little changes that I might not have picked up on if more time had elapsed.
I did enjoy both the book and the film. I didn't cry at either which surprised me, because everyone I know said they had cried and normally I'm the first to cry.
So my advice, read the book then watch the film, but leave a little time between doing the two!