I love how the central themes of the 6th book - namely, friendship, trust, and devotion - are emphasized in this film. It is true that certain details of the book were changed. But what the film leaves out in the way of plot details, it makes up for in character development.
After all, the central actions in the 6th book are completely moot. No one accomplishes any of their goals. Instead, it is how the characters relate to one another that is the main event. The Half-Blood Prince isn't really about the search for horcruxes. It's about Harry becoming wise enough and strong enough to conclude his adventure. This movie does a beautiful job of portraying that.