First of all, it's Harry Potter. It's a MUST SEE, regardless of how good or bad. I thought it was pretty good, even though they left out a lot that WAS needed. Unless you read the book, you wouldn't understand what the "things" were that were attacking Harry and Dumbledore.
They also left out the Gaunt and White Tomb arcs, most of Harry's attempts to follow Draco, as well as Dumbledore's funeral. They weren't NEEDED, although it would have helped explain a lot.
The movie wasn't as dark or depressing as the book. It left out most of Harry's anger with uplifting scenes that were cute and happy. Adding in a scene at the Burrow helped direct the movie back towards the "dark" side. It is still a good movie to watch with a tub of ice cream.