Walking in to the Order of the Phoenix movie, I was a bit worried. The book was big and a bit unwieldy, in my opinion. There were many aspects of the plot that repeated over and over: Umbridge's proclamations, Harry doing lines with Umbridge (no, not *those* kind of lines), the DA training sessions, the slow growing sense of things going wrong.
Also, the book itself was fairly "internal" and "passive" - a lot of the book was characters talking about their situations, and staying in the rooms of Hogwarts, or in the house where the Order stayed. There wasn't much action in the book at all (as opposed to Goblet of Fire which was almost all action).
I was worried that the movie would stick too closely to the plotline of the book and that it would really drag as a result. Luckily, I was wrong. The movie script really lifted the main plot elements of the book and strung them together much more tightly. Repeated elements like those I mention above were dealt with in either a montage of scenes or in one representative scene.
As a result, the movie actually felt quick and tight. It might even have been one of the shortest HP movies, clocking in at just over 2 hours. Some of the new characters in the movie really clicked - both Luna Lovegood and Tonks, especially. The actress protraying Tonks (Natalia Tena) gave Tonks a great sense of being offbeat, fun-loving and thrilled with everything. If I was 20 years younger, I'd certainly have a big crush on Tonks.
I particularly liked the end battle in the Ministry of Magic. In the book, it felt a little clumsy and chaotic but in the movie the first battle of the Order versus the Death Eaters flowed well. The battle between Dumbledore and Voldemort was extrememly good and really gave a sense of big, powerful magic being performed - something that was only hinted at in the book.
Overall, an enjoyable movie. I'd put it up there pretty close to Order of the Phoenix, especially in terms of giving the movie a darker, more adult feel while still maintaining the wonder of the world of magic that is one of the essential aspects of Harry Potter.