If I know that a book is being made into a movie and I haven't read it yet, I may watch the movie...first...so as not to spoil the book. I know that many times a movie just isn't as good as a book.
The only problem with that is that you will already have in your mind what the characters look like.
I enjoy both books and movies and I do realize that sometimes, because of limitations or the way a director wants to go with it....it doesn't always adhere to the storyline completely.
I have been both disappointed and elated after watching a movie that was made from a book.