I never read the book, so personally I cannot comment on any of the relations between the two. However, from what has been said above, the movie appears to be like any typical movie version of a book, changes and misses. I saw the movie just last weekend, and on a personal note as a viewer, I enjoyed it more or less. The graphics was great, the plot turned out well, and I enjoyed the characters and who they portrayed.
I think the changes made was just to make the movie seem more modern. Having them as teens rather than younger kids makes the movie more open to older viewers rather than just a young audience. So, understanding this concept I see the reasons for some changes, though I'm sure others may have been unnecessary. More or less though, the movie was entertaining and I'm hoping to see a sequel in the next year or so.
While waiting on that however, I shall be digging myself up in the books.