I haven't seen the new movie yet. Below are my views of the last 3 films in the Harry Potter series.
I thought Prisoner of Azkaban was the best of the series, although in some ways it was Daniel Radcliffe's weakest performance. It has great music and a great visual style, and a great combination of darkness and humor. I would expect nothing less from a director like Alfonso Cuaron.
Goblet of Fire was OK, but a little disappointing because they took out more than they needed to. I understand quite well that they couldn't have fit everything in, but each scene felt like a mediocre summary of what happened, rather than a well-done film adaptation of the story. Even though it was 157 minutes, which is a fairly long film, it didn't feel like a long film (which is good), but I was left wanting more (which is bad). I honestly think it could have been a much better movie with another 20 minutes or so of material. (And--I'm almost afraid to say this--I think Ralph Fiennes is an over-rated actor who is disappointing as Voldemort. I'm not sure who I would have preferred, but I'm sure they could have found someone who was capable of a darker, more sinister interpretation of the role--someone with more presence.) It's still a pretty good film and does a LOT of things right, but that makes its flaws all the more annoying. I rather wish Cuaron had stayed on as the director because I would have loved to have seen what he would have done with it.
Not counting the latest film which, as I said, I have not yet seen, I think The Order of the Phoenix is the worst one so far. I can't quite remember what it was that I didn't like about it, but I saw it shortly after reading the book and I just kept thinking, "No, no, no--you're doing it wrong." (Oh dear, I sound rather like Hermione, don't I? Well, why shouldn't I? She's one sharp witch.)
It does not surprise me that people are disappointed by The Half-Blood Prince, since it has the same director, David Yates, as Order of the Phoenix. Alas, Yates is also directing both of the Deathly Hallows films. Maybe the fact that he gets two films in which to tell the final story he'll get it right. (Or, he'll screw it up twice as badly.)
I don't quite agree that Half-Blood Prince is the worst book. Although I liked Order of the Phoenix, it was almost intolerable waiting for the action to start. After Voldemort returned in the Goblet of Fire, I was expecting the story of the 2nd war against Voldemort to really begin. Instead, we got well over 800 pages of surly teenaged angst. Don't get me wrong, it was good character development and demonstrated a Judy Blume-like understanding of a teenager's view of the world and their emotional ups and downs, but it wasn't really a story about the Order of the Phoenix, now was it? It was more of a story against an incompetent, Nazi-esque teacher and a budding totalitarian government. Actually, now that I think about it, that's a pretty good story to tell and probably and important one for teenagers to read, but unfortunately the story of the war against Voldemore took a back seat, which is very strange after the dramatic climax of Goblet.
At least in Half-Blood Prince, we get back to the war against Voldemort, and Harry becomes an active participant in the war. On the down side, his participation consisted almost entirely of visiting old memories about Tom Riddle, so it felt rather like being forced to look through someone else's photo album when what you really want to be doing is meeting the people in person and finding out what they are up to and who they've become.
In Halfblood, I liked the Slughorn character--on the one hand he's greedy, shallow, and self-centered--a bit of a gourmand, for lack of a better description, except that he enjoys people as well as food--yet he's a good teacher who is NOT evil. His rejection of Voldemort is admirable, he feels guilty for his role in helping Tom Riddle become Voldemort, and his affection for Harry's mother is rather touching. I also like the sympathetic portrayal of Draco, and I like how Harry unknowingly relates to Snape--in some ways the two have more in common than Harry has with his father.
That's my two cents (and then some). I'll shut up now.