There have been a number of excellent book adaptations I have seen before reading the book. Among them are 'The Reader', 'We Need to Talk About Kevin', 'Never Let Me Go', 'Silver Linings Playbook' and 'Shutter Island'.
Granted, there are occasions when seeing the film does somewhat ruin the experience of reading the book. This was certainly the case for Kazuo Ishiguro's Never Let Me Go, a story which counts on the slow revelation of what's going on. While his writing is still exceptional, my enjoyment of the story was marred by knowing where it was headed.
However, in the case of Lionel Shriver's We Need To Talk About Kevin, reading the book gave more depth to the story, and allowed Eva's voice to come through in a way the film couldn't do. While the story in the film is horrific, the book added depth and breadth to Eva's perspective. Likewise, reading Bernard Schlink's The Reader made me realise just what a great performance Kate Winslet had given in the film.
Earlier this year, there was a lot of hype around 'Silver Linings Playbook', with Jennifer Lawrence winning an Oscar for her performance in the film. Again, I saw the film before reading the book, and really enjoyed its light touch and bittersweet narrative. Friends online who'd read the book before seeing the film seemed incredibly disappointed. Having since read the book, I don't really understand this; it's the same story, with some alterations to fit the screen, and it was always going to be hard to transfer Pat's voice to a film.
In reality it's extremely difficult to adapt a novel for the screen, and it requires a good director with a strong vision. After all, thousands of people have read these stories, conjuring up the characters in their imagination. This is why many can't help but feel disappointed when someone else imagines the story for them, or has to edit the story under pressure of time and money.
So I say, watch the film first, and if you enjoy the experience, you still have a book full of more detail to devour...