I strongly strongly recommend you read the book before watching the film. That goes for the whole series but particularly for this one because this film does not do the book justice. In terms of film quality though this is very impressive, hence the 4* rating. They miss plotlines out which is of course expected but I believe this is the longest book and the shortest film which was a silly choice in my opinion. It flew by dreadfully fast, the first 6 chapters all happened in the space of 15 mins. I wonder if I had not read the book beforehand how much of this film I would understand - not very much I think as the film jumps around.
The biggest plus for this film is that it is visually spectacular, particularly the firework display and the film climax. I was impressed with Evanna Lynch, newcomer, and Daniel Radcliffe who has come on in leaps and bounds since the first film, this is the first time in my opinion when his acting can't be described as shaky. However, Michael Gambon has not read the books and as a result hasn't got Dumbledore quite right, he's supposed to be the height of calm, and polite - the perfect gentleman and headteacher, and he snapped at the students in this one!
In conclusion, this is a great family film but please read the books before watching, or at least watch the series in the correct order!