Loved THE NEVERENDING STORY and THE LAST UNICORN (movies) when I was a kid, and still love them. I didn't read either book until adulthood, but I loved them both as books, too.
THE NEVERENDING STORY is such a different movie from the book, but they both work for me, maybe because the movie is very different. The book is so ... German. Its dark, swirling eddies make my head swim in a way the movie doesn't/can't.
(As for THE LAST UNICORN, I'm not sure why it's considered for kids. The book wasn't, and the movie, though animated, doesn't strike me as kiddish, either. I was obsessed with it as a child, but when I watched it years later with my niece, I kept wondering if it would engage her.)
I'm another one who thought HOLES was a great movie adaptation of a great book. I liked the earlier Harry Potter films way better than the later ones (and loved every single book).
And now all I can think about is all the film adaptations that didn't work for me (A WRINKLE IN TIME as a television movie comes painfully to mind) and all the film adaptations I'd like to see (have there been any John Bellairs movies?).