Robert De Niro is a great choice for the monster, being able to go from pathetic to noble to murderous in turn. Helena Bonham Carter is rather flighty as Elizabeth (I remember her being more docile in the book), but maybe she wanted to capture some of the spirit of Mary Shelley. She's also wonderfully warped when she gets resurrected.
The film is high on melodrama and theatrics -- Branagh seems to want to transfer all the props and sets and costumes he loves from the stage onto the screen. That and the long running time may turn some people off. But it should be approached in the right spirit. The film begins with Mary Shelley in v/o: 'I busied myself to think of a story which would speak to the mysterious fears of our nature'. This is horror set to delight and entertain, as it was in that famous after-dinner story-telling session with all those English Romantics in the Nineteenth Century. As such, it is a triumph.