I thought the movie adaptation of The Hunger Games was excellent.
Making a movie out of a book is always tricky business. There always turn out to be two audiences: some have already read the book, while others have not. You can't make the movie only for one of those audiences, or you'll lose the other.
I thought the makers of the Hunger Games movie did a wonderful job of straddling that fence. The movie explained enough of the details of the book so that the important parts of the story came through clearly, but it also didn't try to explain every detail, assuming (correctly) that people who had already read the book knew how to fill those parts in themselves, and would be pleased to explain any confusing aspects to their friends afterwards.
Collins was credited with contributing to the screenplay. I don't know how much of a contribution she made, in terms of hours and word-count, but it was quite clear that her presence ensured that the movie retained the same overall "feel" of the books.
I thought that the characters were fleshed out wonderfully. Rue was just as I had imagined her, as was Effie Trinkett. I had trouble envisioning Haymitch Abernathy while reading the books, but the movie draws him just right! Cinna was a treat, even if I didn't imagine him like that at all. And Caesar Flickerman! Wow! Yes!
The books, of course, already have an enormous audience, but I think it is quite likely that the movie brought enough additional attention to attract a whole new set of readers, who delighted in the movie and now want to dig into the books and read them all. I hope this happens; the books are fabulous and deserve whatever attention they are getting.
Myself, FWIW, am on Team Peeta; I suppose I identify with him more.