Books vs. Movies

I think most people will agree that books, on a general basis, are way better than the movies that are made out of them. Which is why, when there is a movie based on a book, the people who have read the book usually don't enjoy the movie because of their frustration with the movie's lack of ability to tell the story the way the author did.

A huge example for this one would be Twilight. I don't know a single person who read the book that liked that movie. On the other hand, one of my best friends never read the book and loved the movie.

Now taking this even a step farther, there is only one example for me personally where I liked a movie better than the book, and that was A Walk to Remember by Nicholas Sparks. But I didn't read the book until after I had seen the movie. So is it just that you like whichever version it is that you see first?

I'm bringing this up because I just saw the movie The Last Song and adored it. A close friend (ironically the one who has never read Twilight) read the book and told me that it's a must-read. After seeing the movie though, I just don't know if I'll like it ans am slightly worried that if I do read it, it'll ruin the movie for me. I have the same fear for My Sister's Keeper, which I also saw the movie for and have been told I need to read the book.

Any advise/suggestions?