Now that I've seen Harry Potter and the Half-Blood Prince, here are my rankings of the movies and books at this point.
1. Chamber of Secrets (movie 2)
2. Sorceror's Stone (movie 1)
3. Half-Blood Prince (movie 6)
4. Prisoner of Azkaban (movie 3)
5. Goblet of Fire (movie 4)
6. Order of the Phoenix (movie 5)
1. Deathly Hallows (book 7)
2. Order of the Phoenix (book 5)
3. Goblet of Fire (book 4)
4. Half-Blood Prince (book 6)
5. Prisoner of Azkaban (book 3)
6. Chamber of Secrets (book 2)
7. Sorceror's Stone (book 1)
Isn't it interesting that how much I liked the movies is roughly inversely proportional to how much I liked the books? Part of what influences it is how faithful the movies are to the books, not that I insist on getting it exactly like the book, but until the latest film they were increasingly leaving out significant parts of the books, even parts that help explain otherwise unexplained phenomena or actions of characters. It left a much less satisfying experience, especially if you knew that there was an explanation in the books. Plus a lot of the scenes and entire plotlines that were left out were fun, interesting, and suspense-building. When you consider that the movies were actually getting shorter as the books got longer, it just drives home the disappointment, because there was so much room for more in Order of the Phoenix, the longest of the books but the shortest of the movies.
I expected Half-Blood Prince to be an improvement over the last few, because the book is much shorter than the two previous books, and they were willing to make it a longer movie. I figured they'd be able to include a higher percentage of plotlines and scenes from the book, and I was right. They were. There were still places where they changed things needlessly (most annoyingly at the end where they made Harry's incapacity to act because of Dumbledore's spell into a moral choice not to act). There wouldn't have been a huge increase in time if they'd explained a few things a little better with explanations from the book. The most unexplainable thing was the scene they completely made up that wasn't in the book at all with the Christmas attack. Harry's actions there made no sense. But it was far superior to the three previous movies, which all had major plots missing. What was missing from this was no more than what was missing from Prisoner of Azkaban, but it affected the plot less, so I place it above that. I didn't understand from the movie alone everything that had happened by the end, and I didn't get it fully until I read the book.
This film should be understandable in the most crucial ways to those who haven't read the book, and it's the first one since the two directed by Chris Columbus that that's true of, at least in the most important aspects. But those stories made complete sense in pretty much every way as films, and they didn't cut major plots the way this one did with the private lessons Dumbledore gave Harry all year about Voldemort's past, which they abbreviated far too much in the movie to be satisfying. I also thought they shouldn't have cut out the Professor Trelawney storyline, which both explains more on her prophecies, which will play a big role in the next one (although maybe they'll find a way to put it in that one instead). They didn't explain the Room of Requirement well, why Harry couldn't get in, why it looked different from it did in Order of the Phoenix when he did, and why it looked like what it looked for for Malfoy. They had Ginny hide the book rather than Harry, and I wonder if that will create problems when they need to return in the final movie for the item that in the book Harry sees while hiding his potions book. Leaving out the new Minister of Magic might make it harder to explain the transition for the Ministry near the beginning of book 7 as well, and the absence of the house elves again might create problems for when they have to reintroduce Dobby and Kreacher in the next one.
I'm hoping that the decision to split Deathly Hallows into two movies will prevent it from being any worse than this outing, since there really is a lot to include, and pretty much all of the necessary parts could easily make two three-hour movies if done well (and they're shooting for two to two-and-a-half hours per movie). They're going to have to trim some things, as they did here. I'm just hoping that they choose a little more judiciously than they did with a few things in this film.