Saw Half Blood Prince yesterday as it opened nation-wide, and I have to say, despite the changes from the novel, I thought it was a good movie.
I've seen mixed reviews from fans of the books (here's one from Erika Olson on Redbox) and many dislike the changes and what was left out, but with books as large as the latter in the series, it's hard to get all the details into a 2 1/2 hour movie. Wikipedia has a good summary of the differences from the book to the movie.
There were a few changes that seem to have been made just for the sake of changing something, but I think it's best to hold judgment until we see the finished product, as in the end of the movie franchise's two-part Deathly Hallows. With adapting the movies from the novels, there are a lot of considerations that are made and with similar action coming in the last two movies, the director/producers didn't want the last three movies to seem repetitive. Once we see their vision for the end of the series, our view of Half Blood Prince may change.
One thing that I did note in the film was that the ending felt rushed. No spoilers here, but once we got to the climatic ending, it just happens and boom that's it. This is the best part of the book, and it just seems too rushed on the big screen. My other concern was that the entertaining Half Blood Prince plotline (who was he? how was he so good at potions? why did he choose that name?) was sort of put on the backburner. Even the reveal of who the Prince is was quickly put out there and then dropped.
The major bright point of the movie was its humor. From Ron to Slughorn to Harry, this film was big on laughs. Overall it was a good Potter film, but like those before it, the quality of the books is hard to duplicate on the big screen, and the details of the novels win out as usual.
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