There are hundreds, perhaps thousands, of taglines used every year by production studios, hoping to draw audiences to their films. There is also one that, although it is never used, is the most implicit:
Read the book. See the movie.
It's obvious that the primary demographic for any film based on a previous work is, ideally, the fans of the adapted material. There are, of course, exceptions (how many people do you think saw Da Ali G Show before watching Borat?) but, star power aside, people will largely go to see an adaptation if they have some familiarity with the source material. These same people will also, if an adaptation decides to present ideas or scenarios outside of what the source provides, react with the same vitriol normally reserved for sports spectators.
This is what happened to Harry Potter and the Half-Blood Prince, a film that so many I know walked away from deeply disquieted, and the same film I walked away from with the sense it was the best Harry Potter film since Philosopher's Stone, the first in the franchise. Now that we're a comfortable distance away from the film's arrival in theatres, it's worth looking at again, through more objective eyes.
The most vocal complaint people had with the film was that it dared to present a world not completely in sync with what was in print -- "That wasn't in the book!" is what I imagine many had in their heads as they left the theatre. Is this an adaptation's sole purpose? Yes and no: Yes, because some familiarity should, ideally, be established -- and often is in these sorts of movies, perhaps as a reward for those viewers that want to feel like they got their money's worth.
No, because a film is not a book. No, because it doesn't have to be wholly faithful to the original. (If you want the source material regurgitated, go to the source material.) No, because there is only so much one can put into a screenplay, much of which can't just be gratuitous character camoes for the fans. You see where I'm going with this?
Half-Blood Prince dared to take liberties with the source material and did so without sacrificing the soul of the book, which deals with Harry's further maturation as a character, something that every single adaptation since Chamber of Secrets has failed to do. That film failed, largely, because Chris Columbus wasn't yet ready to let go of the sense of wonder that Harry's world in the first book contained, wasn't yet ready to let the series enter the darker areas of the imagination it needed to traverse.
Alfonso Cuaron tried to make up for his predecessor's hesitation in Prisoner of Azkaban by featuring a radically-revamped Hogwarts and much darker colour palette, but became wrapped up in visuals that deviated too far from the heart of the action (as the infamous "scene with the pumpkins" illustrates). Goblet of Fire favored visuals that spoke loudly and carried a big stick over plot or character development, which seems to be a recent weakness of Mike Newell: his next project after Potter was the misguided Love in the Time of Cholera. Order of the Phoenix was, at last, a step in the right direction, which made me very happy that its director, David Yates returned for Half-Blood Prince, taking a bloated and maudlin book (yes, I said it) and cutting it down to its basic elements while reminding viewers that a film series does not have to be a slave to the books from which it is derived.
Really, Harry Potter is the ideal example for anyone looking to study the relationship between book and film; the only other recent franchises based on a book series were The Lord of the Rings and Twilight, and no-one really complained about them -- something due, I suspect, to how easy it is to do the plotline of Rings (which, frills aside, has one of the simplest plots of 20th-century literature) and the fanaticism surrounding Twilight. With Potter, frankly, nobody's going to be happy, especially with Book 4 and onwards -- there's just so much in there.
Half-Blood Prince waded through the dark jungle that was the source material and found the diamonds, and scenes that have purists on edge contribute to the overall film, which is a prelude to the epic 2-part film that will be Deathly Hallows. Considering how repetitive the endings of Books 6 and 7 are, and how the alteration in the ending of Half-Blood Prince's film -- effectively setting the tone for a darker, bleaker seventh installment -- caused some to gnash their teeth, consider Half-Blood Prince a teaser for future alterations to be made to the source material. I'm sure it would make those watching the Harry Potter films to see near-exact replicas of the book happy to have every film in two parts, but money can only go so far.
Instead, consider what we have, a film that has its faults (doesn't every film?) but is actually the most successful film since the first to preserve the heart and soul of the original story while creating an entertaining experience, even for those that have no idea what a Muggle is.
Yes, there was the scene with the Burrows. Yes, the kiss didn't happen exactly how it did in the novel.
Is it still entertaining, and preserves what Half-Blood Prince is: a very dark coming of age tale?
You bet it is.