Yes, I think it's the best so far. Now that everyone knows that Voldemort is back Dumbledore helps Harry gain insight into Voldemort's past so that they can begin a long dangerous journey to destroy him. Life at Hogwarts continues basically as normal with Snape finally taking up his desired post as Defense Against the Dark Arts instructor as Dumbledore invites fame-obsessed Slughorn to take up post as Potions master. Naturally, Dumbledore has his own reasons for each decision. While Harry and Dumbledore get closer to discovering the key to killing Voldemort the social world blossoms at Hogwarts. Everyone has a love interest, requited or not.

Though there isn't enough time to really develop any of the great tensions of the story, Malfoy is a gem in this film. With an ominous task to complete, he seems to have aged 20 years as the rest of the students are busy navigating teenage romance. Malfoy broods and struggles with his newly-official position in the realm of dark wizards.
Instead of the teen angst of
Order of the Phoenix,
Half-Blood Prince treats the drama between the wizards as mostly lighthearted and humorous, which is a welcome balance to the pending doom of destruction. Harry's bout with Felix Felicis is one of my favorite parts of the entire series. With Ron on the Gryffindor quidditch team the audience finally gets to see some quidditch action that doesn't revolve around Harry and the golden snitch. Ron is a new focal point for the game, along with the greatly improved graphics. Quidditch looks dramatically better. In fact, the entire movie has a wonderful aesthetic appearance. Though not quite a surrealistic as the first version of the film, the color scheme is clear and artistic. The magic regains some of the childish wonder of the earlier films but with much more style--like Dumbledore putting a house back together in the beginning or summoning swirling fire at the end.
Most of the changes from book to film are minor and acceptable with the exception of the destruction of the Burrough, proving Harry's lack of control against the Death Eaters, combined with no immobilizing curse from Dumbledore to keep Harry from fighting back later in the film. Why add an extra scene to make a character point and then remove the major moment that would have made use of that trait? I also wish Harry could have been a little more forward with his feelings for Ginny since much of their attachment is edited out.
I worried that the return to the PG rating after two PG-13 films would mean a less intense film than the story warranted, but I think
Half-Blood Prince may be the most intense of the six. Several scenes were frightening and the climatic scene is both beautiful and suspenseful.
Being one of those special people to dress up and see the midnight premier of this movie I could write paragraphs about the nuances of the story with all the knowledge of the books, but this film also stands on its own. Both the style and story of this film are well constructed, and the lighthearted touches create an extremely enjoyable experience.
If you're unfamiliar with the books, don't spend too much time trying to figure out the meaning of the title. It's ultimately not that exciting or critical to the story.