Not bad, but suffers from a self-inflicted Catch-22.
Since the days of E.T. movie-based video games have been known for one thing above all others: sucking. Harry Potter & The Half-Blood Prince is not without merit, but given the nature of interactive entertainment and the fact that it is not uncommon for a game to provide 40+ hours of gameplay, it is baffling that Harry Potter & The Half-Blood Prince doesn't borrow more from the 800 page novel on which it is based (to legitimately pad the game’s length) while using the visuals from the film on which it is also based, for reference.
Instead, the game suffers from a self-inflicted catch-22. If you have seen the films or read the books, you are bound to be disappointed by the poor adaptation of the story elements in this very short (10 hours or less) game. On the otherhand, if you have never read the book or seen any of the films, you will be completely lost by the plot, characters and even basic elements like the "rules" of the Harry Potter universe and what the hell things like Quidditch are.
To be fair, the longer a game lasts, the longer it takes to make and this game was pushed out the door to meet a simultaneous release date with the film. While it seems like a no-brainer for the game to elaborate on the film by drawing additional material from the book, it would have taken more time for the developers and time is money.
There is nothing wrong with the gameplay of Harry Potter & The Half-Blood Prince. The game actually offers more variety in gameplay than most games of it’s kind. You will spend most of your time making potions in gameplay sequences that have been likened to Nintendo’s Cooking Mama, playing Quidditch, which basically amounts to a Star Fox-ian, on-rails, flight sequence, dueling with other wizards which bares similarities to a simplified fighting game combined with a 3rd person shooter and finally, roaming around Hogwarts castle, exploring and searching for treasure.
Issues with the gameplay generally go hand in hand with foreknowledge of the source material. For instance, when the game began, I knew that one of the first sequences of the book (and the film) consists of Harry Potter and his crew following the evil Draco Malfoy to see what he is up to. So, naturally, I expected the first level would be some kind of stealth level, following an oblivious Draco and spying on his dealings. However, as the game began, those scenes were glossed over quickly by a poor cinematic. The game has to fight the expectation of the gamer and so while the existing gameplay is decent enough, a player familiar with the book or novel will spend more time thinking about what wasn’t included in the game rather than what was. While someone who is not familiar with the film or novel will likely never bother to play the game.
The graphics in Harry Potter & The Half-Blood Prince consist of poor cinematics voiced by imitation voice actors that don't do justice to their film counter-parts, stilted pacing, and a magnificent digital interactive reproduction of Hogwarts School for Witchcraft & Wizardry.
If there is one draw to this game it is walking around the grounds of Hogwarts. You can see exactly where the time of the developers was most spent, as the Hogwarts of the game looks exactly like the films and all of it manages to fit structurally in a believable way. It's really quite impressive.
What's puzzling is that The Half-Blood Prince is a sequel to The Order of the Phoenix and the large scale Hogwarts was originally built for that game. In fact, other reviewers have said if you have already played The Order of The Phoenix, you really need not bother with The Half-Blood Prince as there is really no new majesty to wandering around Hogwarts that wasn't already experienced. So that begs the questions of what did the developers spend all their time working on?
Aside from Hogwarts, the rest of the game's graphics are inconsistent. The character models look very good, but don't hold up to up-close scrutiny, Snape and Hermoine are absolutely frightful. Ginny has plastic hair and Ron's head is too small for his body.
Harry Potter & The Half-Blood Prince is not a very challenging game, which is probably to it's benefit. The game has a number of easy Trophies or Achievement Points for PS3 or Xbox360 players. Some of the later Trophies/Points can be a bit hard to acquire, but are generally attainable. You can see Hogwarts, get some Trophies or Achievement Points and be done in a weekend. Perfect for a rental or bargain bin shopping.