Just got back from seeing this and it was a great film, better than the other two.
Now this isn't going to be a very long review beacuse I'm not very good at reviewing. The film starts with Harry Potter accidentally 'blowing up' Aunt Marge like a big balloon, which is very funny. Harry then gets picked up by the Knight Bus (the big purple bus), but I wont reveal much more of the story.
The Dementors were very good, and very scary (well, scary for little kids). This film is far darker than the other two, wich makes it better IMO. The story starts to get very clever at the end.
One annoying thing for me though was a woman behind me who laughed at anything that was even remotely funny (and some things that weren't even funny or suposed to be funny), really really loudly, she was the most annoying person who I have ever been in the cinema with! But this hasn't got anything to do with the film, so I'll shut up.
Overall, a very good film - 8/10