Overall, The half Blood Prince stays fairly true to the book. The movie does a good job of conveying the evolution of Hermione/Ron's and Harry/Ginny's relationships (rather comically at times).
However, the screenplay fails to adequately explain the decisions Dumbledore makes with regard to Snape and Snape's actions at the end of the movie.
Also, a better view of how Draco's relationship to the Death Eaters has evolved and for what reasons (perhaps not possible in 2 hrs. 33 min) would have been good for the audience to see.
The movie paints a perfect picture of Harry and Dumbldore's experience with the "horcrux".
Good job Warner Bros, I cannot wait for the next installment.