Saw two movies recently. I saw The Half-Blood Prince in the theaters, and saw Watchmen on DVD.
The Half-Blood Prince was a decent book-to-film adaptation. It cut out a lot of plot points, but let's be honest, none of them would work on the screen. As far as movies made about books go, this one actually did well.
Watchmen was excellent. The ending has been changed from the Graphic Novel slightly, but it works. The guy they got to play Rorshach was perfect (to give you an idea: the actor playing Rorshach will be playing Freddie Krueger in the Nightmare on Elm Street reboot next year. Excellent job.) Also, much less glowing blue penis than advertised, which is either good or bad. The sex scene in the middle dragged too long. I agree that the scene was important enough to the plot to be included, but it got old quickly. There are some minor changes, but really I was satisfied with the interpretation.