Just as the books get increasingly mature, so do the movies. No fire-breathing dragons or exciting quidich games. More mature interplay of the characters. Reasonably faithful to the book in story-line and feel. Tweens and younger might get restless, but pretty engrossing for older viewers who have read the books. If you haven't read the last book, it might suffer from lack of understanding of why things are happening as they do. If you've never even seen a Potter movie, then the whole think would be a puzzlement.
James Cameron and Warner Bros spent something upwards of $300 million and 4-1/2 years making the 2-1/2+ hour movie. If you enjoy spectacular special effects, it is a must see. If you are only interested in the story and have already seen Dances with Wolves, there is no need to spend the time and money. No, wait...Since this is a ground breaking film, I think that in a capitalist society, we all have a moral responsibility to do our part to support this effort which will change movie making forever. So buy a ticket, but only go into the theater if you want to see Dances with Wolves in outer space.