Books to Movies - Helping or Hindering Young Readers?

Over the past few weeks, my students have been reading The Hunger Games.  This book is a personal favorite of mine, and absolutely stunning in description.  After personally reading the book, I was really excited about watching the movie.  Unfortunately, it was a monumental disappointment.  I found that there were useless changes made in the movie that did not have to be completely different.  The one that bothered me the most, was the change in the character of Madge...or lack thereof. 
This insignificant change made me think about how my students would or had perceived this movie...if they had seen it yet.  Many of my students already had misconceptions about the book because they had already seen the movie and this was not a surprise to me.  What bothered me was the fact that they had a skewed vision of the story before they even read it.
Yes, this is very common.  Many people see movies before they read the book, but I always worry that young readers will choose to not a read a book just because they have already seen the movie.
While reading with the students, the happiest moment that I had was when my students noted how different the book and movie were, and how the movie was not as good as the book!  I felt that it was a triumph!!  In fact, I challenged my students to read Catching Fire before the movie came out.  It is my hope that they will read the book, watch the movie, and take the time to pick out the differences in the book and the movie.  It is important to me that my students are able to pick out subtle differences and understand why the changes were made.  I only hope that they continue to want to read books even if there will not be a movie made about it!

So I ask...when books are turned into movies, do you think that this helps or hinders our adolescent readers?