Saw OotP last night and its definitely worth seeing, I tried to forget the book while I watched it but since I had just finished reading it again it was pretty fresh in my mind. Some of the things that I missed and thought would be in the movie were Dobby, the protean charmed galleons, the hex'd note from the DA meeting, Ferenzie, and well I woulda loved to have heard "Weasley is our King" and saw a bit of Ron playing Quidditch but in the interest of time I suppose the cuts were necessary. although I really wanted to see those things on the big screen.
So go see the movie and think of it as a movie not a visual book lol...