For me, having just consumed the book, having purchased it in London's Victoria Station, viewing the film version was a lot about marveling at plot conflations and additions. On the whole, I found the movie brighter and less subtle, compared to a darker, more intricate printed work. On the other hand, the film had its own semaphores and semiotic twists -- I'm not trying to dismiss films as intrinsically inferior to books.
For me, the novel was effective in sucking me in to some character's inner life (the opening sequence), seducing me into a reality according to that viewpoint, and then abruptly changing viewpoints, and looking back from behind a different pair of eyes (his wife's). I'm talking about Sandy, not Justin, but when we move to him (and more or less stay there), once again, the outlook so drastically changes. We get access to new secrets, and closer to the world of Friends.
The film version adds a lot of good imagery, and if you want to go back to the original plot, borrowing these wonderfully rendered characters and scenery, that's an option. Or, to put it another way, if you saw the movie first, and now wonder about reading the book, I'd say "go for it."