I found this game quite enjoyable... while it lasted. The trouble is that the storyline only took me about 7 hours to complete, and while there are other things to do (collect badges for flying/duelling/potion-making, collect all the Hogwarts crests) I am disappointed that it isn't longer because I have almost finished it after about three or four days on it.
I found most things to be too easy (I only replayed a duel once, only replayed flying twice) but there is always the option of going back to a club later and improving your score.
The graphics are really great in my opinion and I enjoyed the lack of loading screens and being able to explore Hogwarts - although I thought a point was made about being able to walk all around Hogwarts, this is not entirely true as not all of the floors are in the game whereas I believe they were in the Order of the Phoenix (OotP) Game, and I couldn't find the Divination classroom although Harry doesn't take the subject so it isn't really necessary in the game, but it would have been nice to visit.
I also thought it was a shame that you can't do the four lessons (Potions, Transfiguration, Charms and Herbology) and get a grade like you could in OotP.
But it is a fun game. Potions making is different now: it's more complicated with step-by-step instructions for each potion which are based on turning the potion different colours - by heating, mixing or adding ingredients. It made good use of the PS2 analogue sticks without being too tricky. Quidditch consists of the normal flying through rings but there are training dummies to hit for a time bonus. Duelling has been also been improved with more freedom of spells to use (both for the player and the computer) but I will say again that it is not very challenging.
Overall I am glad I bought it but I think I may prefer OotP simply because there was more to do. A must for a hardcore HP fan (like me) or children who would probably enjoy it, but possibly not good value for money on newer platforms such as the Wii or PS3.