For those who have read the book, you should be able to get the most out of the game.
Harry, Ron and Hermione are back at Hogwarts once again for another year. A new teacher called Professor Slughorn gets his old job back as Potion master and Professor Snape takes the Defence-against the Dark Arts. Harry and Professor Dumbledore also find out more about Tom Riddle (Voldemorts past). If you've read the book you should probably get the most out of the game more then to the film. But if you've only seen the films you might get a little lost on story if you haven't read the book.
Gameplay: If you've played through Order of the Phoenix on Playstation 2 or 3 the controls feel pretty much the same. You'll be using the analog sticks to cast your spells like Wingardium Leviosa you can make the object float, or casting Expelliarmus during wizarding duels. You can explore Hogwarts without any loading screens at all which is good for most gamers if you've used to free-roming games in the past. If you've played through the Order of the Phoenix you used the mauralders map to know where you need to go and you'll see footprints on the floor telling you which way to go, but this time Nearly-Headless Nick will be your guide person in order to get around Hogwarts. Yes Headless Nick did appear a lot in the books and in the first 3 games as well but I guess that makes most Potter fans happy to see old charcters again.
There are mini games as well, where you can make potions in sessions or in Potion clubs. Each potion you make you would have to get the right colour and mixing to make the potion. If you don't there will be a lot of smoke and you can easily get rid of it by pressing the shoulder buttons L2 and R2, and plus you be timed as well on how quick you make it. Another mini game is Wizarding Duel where you can battle other students and through out the main story as well where you'll be facing some bosses but they can be really easy to beat if you keep firing spells a lot by using the right analog stick. There are 5 spells you can use in duels.
You can also play with a friend if you head to versus mode at the main menu, where you can select your favourite charcters like Ron, or Crabbe or Luna. But I guess that the 2 player wizarding duels kind of make it feel like a fighting game but using wands as weapons. There is no online mode the wizarding duels which is a shame for Hardcore Potter fans out there.
Quidditch makes a return in the Half Blood Prince the last time we saw Quidditch as a mini-game and task mission was back in 2004 which was the Prisoner of Azkaban. Where you will be playing as Harry as he try's and catches the golden snitch (Which is a small golden ball where seekers have to catch it). In order to go faster you have to fly through the coloured stars to go faster but if you miss any your time will run out and you would have to start again. You can't use the sixaxis controls if you're flying a broom, you can only use the left analog stick to move, it's a shame because it would have been a nice idea for the game.
Graphics and Sound: Like I said nothing has changed from the graphics they pretty much feel the same as the Order of the Phoenix was back in 2007 if you've played it that is. But the charcters have minor detailed looks and expressions which are a little rushed trying to release the game at the same time the film is released. The best part for most Potter fans would be exploring Hogwarts and the school grounds as usual seeing Hogwarts for what it is and seeing the world a different place in what you would see in your normal schools.
Sound is pattern of on voicing acting because most movie-based games you would have the main actors from the films doing the voicing for the game. Some main charcters still have the same voicing as what you would hear from the main charcters like Harry, Ron and Hermione but there not 100% perfect. But for charcters like Snape, Dumbledore and Nearly-Headless there a lot different from the movies which a lot of people loved, then again I guess that the game does try it making perfect as what the book was. But the voicing for Crabbe and Goyale are a little funny in the game where they try and block Harry's way and saying stuff like "Well if it isn't the losers from Gryffindor, there …. Losers" it's funny because in the books they were Malfoys bodyguards.
Music has always been the same in the other Potter games with magic and fantasy putted with a bit of John Williams's music composing in the film and games as well.
Overall: Most Potter fans might get the most out of the game, if you've found the film a big disappointment on plots and endings. There are trophies for Half Blood Prince if you want to get 100% but most likely you'll get over 50% once you've beaten the main story which can easily take you up to 4-6 hours to beat.
But there's plenty of things to do before and after you can collect Hogwarts crests at different areas, floors, walls in Hogwarts the more you collect you'll be able to unlock more charcters for wizarding duels, more health and your spells will be more powerful to use.
Some people might feel a little put in of but the game being so short meaning that you might be better of reading the book again or you could rent the game if you can't be bothered paying £30-£40 for the game. But when the next Potter film is made which is Deathly Hallows and which is going to be in 2 parts but how will they make the game for it not being set around Hogwarts? Potter fans will just have to wait and see.