Just buy the movie (enticed me to read my first HP novel, "Deathly Hallows" cuz I couldn't stop with the end of this movie) - the sneak peak isn't worth a darn thing. However, if you want to learn something of Jo Rawling, there is an insightful documentary of a year spent with her as she takes on the completion of the series with "Deathly Hallows".
What's great about it: The story of course and this one portrays the inevitable twist quite well
What's not so great: Issue isn't the movie, not worth the extra $$ for the extra$