First of all, let me say that, contrary to what one of the other reviewer said, it is my experience that those who loved the first two films, are often not too fond of this one, and the other way around. I have heard people who adored the first two films be rather rude about this one, actually.
Prisoner of Azkaban is not fluffy, it has no exaggerated emotions (they've even managed to tone Ron down. Well, a bit anyway.), has no cringeworthy scenes - although there could have been one, but Gary Oldman pulled it off -, and has a great (British) sense of humour. Cuarón manages to convey Lupin's sense of fun without needing him to shoot a wad of gum up Peeves's nose, several blue birdies meet a sticky end, and even in the thick of the action Harry comes out with the odd dry remark.
It is odd that some say the film "butchered" the book, since, in fact, parts of it have been taken word-for-word from the book.
This film is not as spoon-fed as the other two films. You have to think (although having read the books does help). There is plenty of action -the only bit that had my 5-year old son bored is the Shrieking Shack scene, and believe me, he gets bored easily - and, well, it's generally a cracking romp.
My son's opinion: he no longer wants to see the first two films, this one has spoiled him forever.
My opinion: I loved it!
So: a good one for young and old(ish)