I've seen worse costume dramas - The Other Boleyn Girl comes to mind - but I was lukewarm on The Duchess, which stars Keira Knightley, Ralph Fiennes, Charlotte Rampling and that cute dark-haired boy who sang Lay All Your Love On Me in Mamma Mia!
If you're a fan of period costume drama it should hold your interest, but ultimately, I'm not sure it adds up to much. You can read my review here and judge for yourself if it's worth your time. On the other hand, who am I to complain? At least I didn't have to see Nights in Rodanthe, also adapted from a book.