I went with two friends to an opening day show, and overall I am pleased. The only negative comment I have is this: it was really rushed. That's quite a statement because while The Order of the Phoenix is the longest Harry Potter book to date, it is the shortest movie. In hindsight, I wish the filmmakers had made it slightly longer so as to flesh out some of the abbreviated scenes.
My greatest disappointment was towards the end. [If you don't know the ending of the movie, and you don't want to just yet... skip this paragraph] In the book there were so many fascinating, strange rooms at the Ministry of Magic that I was looking forward to seeing, and the movie left out a hefty portion of them. Among the missing were the rotating-doors room, "brain room", and the scene where one of the death eaters gets his head caught in a time loop - all very memorable to me.
However, the scenes that were cut made way for a very well-done battle scene at the end. Great special effects, great acting all around.
The jewel of this movie is Professor Umbridge, played by Imelda Staunton. Physically, she barely resembled the character I had envisioned from reading the book, but she won me over with the subtlety with which she portrayed her. Just the quiver of her cheeks spoke volumes. The cheery-sinister mix of personality reminded me of a high school English teacher I had that would speak in a happy, lilting voice, but would stare you down with an ice-cold glare if you crossed her. Not that I did that on a regular basis... not intentionally anyway.
Overall a great experience. Simply too rushed. Like going down a water slide. Just when you are getting to enjoy it, it's over.