I just saw Harry Potter 6 yesterday. I must admit that I was pleasantly surprised by the movie. 4 and 5 were not great movies; the pacing was too fast and too much dialogue had been cut to make the movie fit the 2.5 hour format. With Harry Potter 6 they managed to redeem themselves and make the movie work.
There are a few notable plot changes in the movie; most crucially there was a battle inserted in the middle and one removed from somewhere else. Some minor details here and there were changed, sometimes in ways that seemed odd, and sometimes in ways that violate the canon of the story, but nothing truly important was tampered with. Most characters appear without introduction and if you didn't recognize them from the books or previous movies you won't be able to recognize them now either. But aside from that the movie progressed fairly well and in accordance with the original story.
This is perhaps the best instalment so far since the first movie, which was the most true to the book.
One thing that bothers me is that the Death Eaters seem to have the ability to fly. In the 7th book they point out that Voldemort can fly and everyone is surprised by that. Oh well, it's not an important detail.
One thing I found funny is that certain actors have aged A LOT since the first movie and others don't appear to. This makes Rupert Grint, who's 21, look really out of place in Hogwarts, where the incoming students are 11. And some of the teachers in the school have aged a lot and I worry that they won't be around to play their roles in book 7. Maybe the director will film the end of that book sooner, since the teachers play a bigger role at the end than at the beginning.
Anyway, overall I recommend this movie to all fans of the books or the movies. It won't really disappoint; as far as movie adaptations go it's one of the better ones.