I really enjoyed it. I'm pretty critical of the movie adaptations in general and I didnt't particularly appreciate any of the others, specifically GoF (too many important details either changed completely or missed out altogether) but OotP did it for me. Whatever I've been looking for in the movies was there, it had It.
As a few people have mentioned, the 5th book was sort of long winded in the first place and I think the movie portrayed most of what mattered. I especially liked the fact that there wasn't all that much action until right towards the end because that's what happened in the book.
8/10 from me, and that's saying something 🙂
One thing I would have really loved to have seen was Firenze taking Trelawney's place as teacher of Divination. That was a brilliant part of the book, it's a shame it was cut.
Also, I just want to point out to those who say it's a good thing they read the book before seeing the movie; surely that's a given? It really bothers me when people suggest they don't need to read the books because they can just watch the movies. Big mistake in my opinion 😬