Looking at it as an individual movie itself, I found it a bit lacking... It is not a movie that the uninitiated muggle should go to see. Without the background of the other movies or books it is very lacking.

But, for the well initiated, you can easily see that it is laying the groundwork for something very big to come. I totally agree with walterrick's comparison to Star Wars: The Empire Strikes Back; that being said, I still think SW:ESB stood on it's own much better than Half-Blood Prince did.

Still, I enjoyed seeing it, and look forward to the next two movies.