To keep it shorter than long:
I liked this third installment in the HP series. I think it captured the energy of the third book and tried hard to keep the essence of the plot. The third book is so damn huge there's no way to put it into one entire movie, so cutting out elements and rearranging others was neccessary to keep the momentum going.
Hogwarts grounds weren't how I imagined them to be, but I did enjoy the look of it nonetheless. I was kind of annoyed that the entrance to Gryffindor commons was right where anyone could see it rather than being secret like it is in the books. Some people had commented on the Whomping Willow being too small or something - I thought it was perfect and looked better than the one in the second film. Lessee... I liked how the arguments between Hermione and Ron over Scabbers and Crookshanks was more incidental - that way, Scabbers' true identity would be more of a suprise to non-book-readers. That and the fact that arguments like that between friends ARE often background things as compared to overall HUGE things in life.
I enjoyed the effects in the film - the Mauraders Map was well done (I watched all of the end credits to watch all the little feet, though it got a tad monotonous after awhile). The Patronus spell was FANTASTIC and actually gave me the chills when I saw it the first time by the pond/lake. I liked Buckbeak and thought that Lupin's werewolf form was well done, though a bit more emaciated than I'd imagined.
The acting was also superb - especially the three kids, they've really grown up. Lupin was great, too - that actor hardly ever gets to play good guys. I know that the new Dumbledore was supposed to play Dumbledore HIS way and not try to be Richard Harris, because you can't replace a person. But I still just like Richard Harris' Dumbledore better. *shrugs*