It followed the main plot of the book, however they did leave out some details that would have better helped if you have not read the books. They should have focused more on the text book helping out Harry, and built it up more to who exactly was the half blood prince and how they tried to figure it out, and less about teen romance. The movie was pretty simple until the fight scene but that's where the book hit it's climax. They also should have focused more on the "good Snape, bad Snape" that wasn't nearly as portrayed as it should have been. All in all it hit the main points fairly well!
First, You must read the books before you see the movie or you'll be lost for the most part. Other than that... The best movie of the year if not a few years. I'm so Twilight obsessed I was afraid the movie wouldn't follow the book, but it over exceeded my expectations! I'm more obsessed with Rob Pattinson, and he heats things up just nicely as well ;)... Everyone enjoy all things twilight, but remember to READ THE BOOKS FIRST!