If I could have rated this film lower than 1, I would have. I agree completely with Freudella's commentary (which see), and I don't understand how she could give it even 5*.
The events of the book are technically correct in the film, but the essential charm, agony, poignancy, and sense are completely missing. I do wonder how I would have felt about this film if I'd never read the book; I think it would have seemed merely baffling and pointless. As it is, the disappointment is deep.
I love a good "quirky" film, but for all its quirkiness this film is (paradoxically) both bland and nasty. No believable context is provided for the characters' eccentricities, which hang suspended out of time, place, or background. Whereas the book amazes with its revelation of the tension between Cassandra's innocence and growing knowingness, and the preservation of innocence within knowingness, the movie makes no space for her innocence at all. Contrast that with, say, Hudson's MY LIFE SO FAR.
Adolescent romance is an expression not just of sexual drive but, even more, of the need to make sense of adult life. Dodie Smith understood this. These filmmakers did not.