It was clear from the commentary that the screenwriter did not understand or appreciate much of the book she was adapting. Almost any adaptation has to change or leave out parts of a book, but this screenwriter added unnecessary melodrama and destroyed the humor of the original. She also claimed scenes were true to the book when they weren't and made changes that didn't make sense.
The worst was that she made a big deal about not changing the ending, saying that Dodie Smith (the author) had been offered half a million dollars by book-of-the-month club to change it and she (the screenwriter) felt that if Smith resisted that much money it was important the ending stay the same. And she changed it! She changed it in a different way but the change was just as bad, arguably worse.
She also changed some scenes in a way that made them not only contrary to fiction, but also to the real era in which the events of the movie are supposed to take place. Many movies do this, but I wish they wouldn't.
On the plus side, the scenery was beautiful (though wrong for the supposed setting) and the acting was good for the most part. It was too bad that the story was changed enough that Neil and Simon behaved bizarrely and inexplicably. Neil especially was a strange caricature of an American, but given what he had to work with, I don't suppose it was the actor's fault.
I'm sure I would have liked this movie slightly more if I had not read the book, but I still wouldn't recommend it to anyone. The screenwriter's plot just doesn't make sense. Maybe no screenplay could have adequately conveyed the character's motivations and the plot only makes sense in book form, but this attempt is truly terrible.