I prefer to see a movie before I read the book, usually. That way, the story is (hopefully) fresh, and the book shores up the story, or explains some stuff. If I read the book first, the images I "saw" in my mind are almost never equaled on the screen. For example, "The Puppet Masters" disappointed me, but it was one of my favorite Heinlein books.
However, GWTW was MUCH more interesting in film than on paper. I've never finished the book. I didn't even know what "CSA" stood for. I thought it stood for "Cub Scouts of America." Maybe my age kept me from understanding the book, but I've never tried the book since.