This is where the Harry Potter series suddenly gets some great direction and awesome cinematography. They also never do explain what's going on as well as they do in the book. Hello? Why do these adult people know about the Marauder's map? Oh, you don't need to know that unless you're reading the book. Of course.
They could have stretched this out over a few movies and actually stuck to the plot, but instead they made up a few things that were completely out of character and blurted out secrets that took whole books to expose. If you haven't read the books it'll make very little sense and if you have read the books you'll be annoyed by it (and not just for the obvious "They're not dark-skinned" point either).
If you try to forget any relation to the book and just enjoy the movie for what it is, you'll find it's quite bland (it does have a rather cute song, though). Goro Miyazaki is using Studio Ghibli resources, talent and reputation, yet not quite living up to it himself.
The book is far better, but it's been put together fairly well and some moments are actually very funny. It's a good story for pre-teens to learn to get out of their own heads and think about other people's problems on occasion.