So there's this new website called http://readit1st.com/ that's all about the idea that it's better to read the book before the movie version comes out. Now I support this idea, although I don't always follow it. There are plenty of times where, because the movie is coming out a lot of people check out the book from the library so they can read it first. Again, I support this. However, it means that I don't get to read the book first unless I go on a reserve list, which I think takes the fun out of it a little (Not that I won't use reserve lists, I just think it's less fun than scanning the shelves). That's why I like that the pledge is to read the book first when possible, not always (There's also an alternative pledge about just reading the book whenever).
I think this pledge to read the book at some point, though not necessarily first is what's most vital about this website. So many people don't read who should and some people miss out of watching great movies because they were waiting to read the book. Speaking as someone who's read the book before and after it's an interesting experience.
When I saw the Harry Potter movies I spent a lot of time criticizing what was done wrong (like here). It's hard to not look at what's missing when you see an adaptation of your favorite books. But at the same time, I've also had the experience of seeing the movie first and then reading the book. For someone like me who is quick to criticize a movie for being "wrong" (though I've gotten better at differentiating between good movie and good adaptation), sometimes it's more enjoyable to see the movie first. Since books tend to be more in depth rather than complaining about how much was left out, I can enjoy the movie and then afterwords learn how much more was under the surface. I'm not going to lie, sometimes I'll read a book and start getting mad that things were left out of the movie, but in general if I read the book second I'm more accepting of changes in the story.
I still try to read the book first when possible, but I don't want that to dictate my enjoyment of the movie.