It very losely scans over the plot of 'The Half Blood Prince,' concentrating on key-plot sequences rather than detailing explenations and what-not.
The graphics aren't too bad, although some sequences and details looks a bit ... goofy ... at times. Other than that, game play is better than the previous game, with a lot of different levels and goals to achieve.
The castle is much easier to digest and explore this time round, with Nearly Headless Nick as an 'easy-to-follow' guide if you get lost, rather than the annoying, vanishing footprints from the last game. There are a lot more places to discover inside Hogwarts that were non-existant before. (You, also, get a brief flight around The Burrow, and an encounter with some weird looking inferi-zombies in a cave.)
Quidditch tends to get a bit repetitive, but it's quite fun nontheless. Flying is easy to get used to after a bit of practice. The only problem is getting the hang of flying through the goal-hoops.
Duelling is fun, but it can be a bit easy when you get the hang of dis-arming your oponent (which doesn't take long to master at all.) There are a selection of new spells that create different reactions. Two-player duelling is great fun. You can play a selection of character's from the franchise and duel your opponent.
Over all, a pretty easy game. Didn't take me any more than a few hours to complete, and I'm not a huge gamer. Still, it's really fun while it lasts, and it's nothing I wouldn't play again just for the fun of it.