It seems only fair to give a little background on my opinions about the Harry Potter series, books and movies, so you'll know where I'm coming from when I tell you why I liked the fifth movie, Harry Potter and the Order of the Phoenix. I've read all the books by J.K. Rowling (except the seventh, of course, due later this month) and have enjoyed them overall -- but my least favorite was the fifth book, the one on which this movie is based. I thought it was at least 100 pages longer than it needed to be, and I grew irritated with the overload of Sullen Teen Syndrome from Harry. You have no idea how relieved I am that the movie successfully eliminated many of these difficulties, to the point where I found it an improvement on the book.
I've also seen all the movies, most of which I liked well enough but found unmemorable, with the exception of the third movie (Harry Potter and the Prisoner of Azkaban), which had better character development. Yes, I am firmly in the "Alfonso Cuaron rocks" camp, as opposed to the "Chris Columbus needs to return" camp. The previous movie, Harry Potter and the Goblet of Fire, seemed so rushed and crammed in an effort to condense a thick book into a single movie that I didn't have high hopes for this film. Fortunately, Harry Potter and the Order of the Phoenix was able to pare down the storyline to its essentials without making me feel like we were missing anything critical.