I re-watched Atonement the other night. Romantic, tragic, beautifully filmed. This is an odd story - usually, if I like something, I like it in any format (if well done, of course), for instance, I love Lord of the Rings in any form... book, film... it'd make a great opera, I think. Ditto tales of King Arthur. The Arthurian stories work as: epic poetry, Idylls of the King; as a musical, Camelot; as several very different novels, including The Once and Future King and, focusing on Merlin, The Crystal Cave and its sequels. (Avoid First Knight, a terrible movie - Richard Gere as Lancelot? As bad as Tony Curtis.) A good story keeps working.
But Atonement... the switcheroo ending is powerful in film. In the critically acclaimed book it just... makes me mad. Furious. The author breaks the unspoken agreement between writer and reader: "If you, the Reader, invest your emotions in my characters' fates, I, the Writer, promise to make it worthwhile." It's okay for a writer to harrow a reader's feelings if that's well done and satisfying, but it's not okay to cheat. A reader invests a lot of time in a novel, while a movie watcher only invests a couple hours, maybe that's why I was willing to be shocked but satisfied(ish) with the film version, but royally teed-off by the book?