I skipped my norm - rereading all of the books before the new movie - and am very happy I did. In the years since I read the Half Blood Prince, I forgot almost everything but the major plot line so I wasn't upset about "they left out this! they left out that!" I'm sure there are enough other reviews on here to tell you about the how the movie unfolds but i thought I'd just pass along my tip to moviegoers.
One other comment: the cinematography was beautiful!