I first read Watership Down when I was 15 or 16, and then I saw the movie. That was the moment I learned that a film can never be as good as the book. Never.* My imagination is always going to be better than a 2-D representation (or 3-D, as current fad has it) of someone else's imagination. See the movie first, and then read the book if you must. Or read the book and see the movie with the realization that parts of it will be disappointing. My current Netflix movie is the first part of The Thorn Birds mini-series from the 80s. I know it won't be as good as the book, but sometimes it is nice to see a representation of the characters, and I would love to see the setting. Uh oh, I just Wikipedia-ed it and it wasn't filmed in Australia! See, I'm disappointed already.
*Disclaimer: I am aware that one should never use never. I admit that there may be the rare film that is better than the book, but that is probably because the book is less well-written than the screenplay. (Terribly polite way to say the book sucks, isn't it?)