There is a lot to like in this, Kenneth Branagh's stylish and ambitious take on Mary Shelley's legendary story, and, while I came extremely close to giving this a mild recommendation, I'm ultimately just going to have to not really do that, and give it a C+. See it if you really want to, but rent it or catch it on tv or something.
Visually, the film is pretty good. Some of it might look a little fake and dated, but that may have been the point. I liked the art direction though, because all the period details just look awesome and like someone put a lot of time and money into them. The really main issue here is with the specific story and script. At times the movie follwos the book fairly well, but at the same time also takes a lot of liberties and diversions. Everything is a jumbled mess and seems pretty disjointed. The direction isn't too bad, but Branagh has done better work.
The casting is where this film is really interesting. Besides directing, Branagh takes the lead as Victor Frankenstein. He's actually a pretty fitting choice and embodies the role well. Helena Bonham Carter also gives a decent performance (and looks great) as Elizabeth, Victor's adopted sister/fiance. The real interesting casting is that of De Niro as the Creature. He kind of sticks out here. On one hand, he's good with doing the anger thing, and looking creepy, but on the other, he's really hammy, and it's hard to take him seriously. They should have gone with a lesser known or something, because this feels more like purposeful stunt casting, and not something genuine.
This film has some good sequences, but, much like De Niro's participation, things are a mess. Perhaps it's a bit too manic, bombastic, and ambitious? It's better to have more ambition than not, but still, this is a loud and exhaustive affair. It is watchable, despite not being all that good, and, the hamminess aside, it won't kill you to see it even though you'd be better off doing something else. I mean, after all, it's not the worst take on Frankenstein, so take that as some kind of praise if you want.