The book (by Dodie Smith) is better. It's usually so, as it has the space to develop the characters, set the setting (as it were) and be more satisfying.
But the movie for me ranks 8/10. Adaptations are always difficult, but here an harmonious compromise has been reached.
The acting is not uniformly good, but each performer gives the best of itself. Particularly Tara Fitzgerald, Romola Garai, Henry Thomas and, yes, Marc Blucas(if you've only seen him on 'Buffy' you're in for a treat).
All in all a wonderful, old-fashioned with the odd touch of XXIst century romantic comedy.