As a fan of the book, what irks me about this film's reception is that other fans expect it to be a perfect reflection of the book--it's not, and it shouldn't be.
The book Harry Potter and the Half Blood Prince was a segueway into the Deathy Hallows, and the movie effectively creates that bridge.
The acting of this film was superb: Michael Gambon reveals a softer, more accessible Dumbledore; Emma, Daniel and Rupert mature along with their characters; and one of the the most interesting and moving performances of the film came from Tom Felton, who plays Draco Malfoy.
HBP was inspiring, dark, funny and endearing. Don't come into the film expecting a pedantic retelling of the book, and it won't disappoint.