I took my kids to see Harry Potter and the Order of the Phoenix today. I've read reviews that say that it's not a film for small children, and no, it's not. That's what the earlier films are for. Let's be clear: this is the fifth of a series. Unlike the early books, this is dark, it's moody, it's violent in places. Harry Potter and the Sorcerer's Stone was written about an 11 year-old, at a reading level appropriate for 11 year olds. This was written about a 16-year-old and an appropriate reading level. That said, my own 11 year-old children enjoyed the movie, and were quite well prepared for the darkness of the film by virtue of having read the book first.
If you want to get full enjoyment out of this film you really have to read the book first. My older brother (who went with us) had not. He found it to be disjointed and confusing. I can certainly see his point. Nevertheless, when you've got a limited amount of screen time, and 4 years of backstory you can't be expected to get the audience up to speed. Watch the other movies, read the other books, then see this film.
The rest of this post is for people who've read the book. If you haven't then it won't make sense. Stop reading now.
For the most part I was satisfied with what they chose to keep and leave out. As I said, they've got a limited amount of screen time. This was a big book. And it's so full of good moments that it's easy to get caught up in all the things that were not in the movie. For instance, I wish that Fred and George Weasley's exit had been a bit more like the book: as a result of a confrontation with Umbridge, rather than just out of the blue. I miss the swamp. I miss Peeves. The movie version of their departure seemed... insufficient. I wish Tonks had more screen time. I wish Kingley Shacklebolt had more than one line. I miss the Sorting Hat's new song. But the movie was long enough and we can't have everything.
On the other hand, the moments that were included were well done in themselves, even if somewhat clumsily stitched together. Harry's speech to the prospective members of "Dumbledore's Army" in the Hog's Head tavern was absolutely riveting. And I believe director David Yates was extremely effective in getting across the message that Harry is the best Defense Against the Dark Arts teacher these kids have ever had. That's what I watched the movie for in the first place... that and the death of Sirius Black, and Dumbledore's confrontation with You-Know-Who... all nicely done.
The casting was superb. Delores Umbridge was exactly as described by the book, and just as sweet and hateful. Luna Lovegood was exceptionally well cast (she was played by a rare newcomer in this cast of notables. And thinking of casting... when was the last time you saw a movie with such an incredible cast of notable actors, some of whom only got a minute or two on the screen? C'mon now... Michael Gambon, Ralph Fiennes, David Thewlis, Alan Rickman, Gary Oldman, Emma Thompson, Maggie Smith, Robbie Coltrane, Helena Bonham-Carter... a tremendous torrent of thespian talent.
The one real complaint I have was that (in my theatre, at least) the movie was too dark. Not the themes or mood... there wasn't enough light on the screen to make out details that should have been visible. Sadly, at this moment I don't know if it's the director's fault or the projectionist's.
Summing up, Harry Potter and the Order of the Phoenix is definitely worth seeing, but only if you've got the backstory. Read the books or watch the preceding films, then watch this.