You really get to explore Hogwarts in this game. Secret Passages, classrooms, common rooms, dorms, they even have the dock where the 1st years get off their boats after the traditional ride across the lake. Spiders flee the school, and the staircases really move.
The game follows the movie more than the book, and I was rather disappointed that there was no sign of Dobby.
The most challenging part was probably sneaking around unnoticed. Or getting out of the Forbidden Forest. The bosses weren't very hard and the basalisk was just laughable.
The worst part was probably just waiting for it to load. I've never played a game that has had to load like this before. It take you 5 minutes just to leave the library and come right back!
Exploring Hogwarts and Diagon Alley is reason enough to buy the game. The fact that it's playable as well is awesome