This includes spoilers, so if you haven't seen the movie yet, you might want to go elsewhere.
I re-read the Half Blood Prince a few months ago so it's still pretty fresh in my mind. I went to see the movie today with high hopes. The special effects were certainly spectacular. But the telling of the story got lost somewhere along the way. Many parts were rushed through to the point that I'm not sure how someone who hadn't read the books already could have followed the plot at all.
Also, they made Professor Slughorn such a nice guy when in the book he was an arrogant buffoon that Harry Potter tried to avoid.
But perhaps the biggest disappointment was the final battle. In the book the whole school gets involved with the Death Eaters as Tonks and Bill and the rest of the adults on the side of good get into an epic battle inside Hogwarts. I couldn't wait to see how the movie would handle that battle. Well, they handled it by omitting it completely.
They went from Dumbledor falling from the tower, to the Death Eaters escaping from the grounds. I suppose they did that to simplify things and keep the movie the right length. But seriously, if I was the writer, I would have cried. We can only hope that the epic battle is in the director's cut but I'm kind of doubting it.