Sure, the best thing in this movie is a story: girl didn't understood situation and broke her sister's life.
What I like and hate at the same time here - the final. We know already that in real life both lovers died but the movie shows us the end of the book, where these people are together and happy. That time you understand, it's not true, it never happened, you'd like to say "stop it, I don't want to see it! It's not true!" But you can't because it's a movie and you can't stop it.
Another good thing - views of London. Let's say, "picture"'s really good here, you can almost feel the atmosphere.
I can't say acting is amazing - Keira Knightley didn't impressed me at all but James McAvoy was just great! I'm sure, he's one of the best actors now.
It's a must-see movie. But I wouldn't recommend it to see more than once.