For those who enjoyed the book first, this film seems strangely wrong. The filmmakers are too greatly influenced by a combination of the Merchant-Ivory films, all those wonderful Masterpiece Theatre/BBC historical British dramas and a sort-of light, airy type of film-making that's completely out-of-place for this story.
This family is living on very hard times and I don't think that's shown properly. In the book, Topaz is a much stranger character (in the book, she reminds me of a blonde version of perhaps, Morticia Addams(!) and Mortmain should not just be moody. In the book, he's much more odd...and interesting.
I guess they got Rose and Cassandra right, but there's none of the poverty and hopelessness that's so clearly drawn in the book.
Perhaps this one of those books that simply couldn't be properly adapted nearly 40 years after it was written.