A friend and I caught Harry Potter and the Half-Blood Prince on opening night and I have to say I enjoyed it thoroughly. I figured it was worth a (spoiler free) review for those of you who are thinking of seeing it. Which, really, should be most of you.
First off, if you're going with the kids, use your judgment, there are some genuinely creepy scenes, including the one in the cave full of inferi (zombies) that is shown in the advertisements. Creepy as it may be, though, I think most kids are OK with it, just be prepared to jump out of your seat at least once (trust me, it doesn't help that you know it's coming).
The Potter movies, like the novels, get progressively creepier, darker and more violent as the series progresses, which is appropriate to the story, and Half-Blood Prince is no different. However, another element thread that runs very strongly through this movie is the character's growing interest in... well, each other. There is plenty of awkward, painfully intense teenage romance going on. Younger viewers will relate, while older viewers will... probably relate too, but on some level cringe a bit and think "my god, that used to be me."
Ron gets the worst of it, mostly because he gets what he thinks he wants, and steals the show for pretty much all of the non-combat scenes. Harry and Hermione get their share of the angst though.
The quality of acting is, generally, superb, as it has been in most of the Potter films. Dan Radcliffe does fairly well with what he's given in Harry, although by this point in the series Harry is probably the least interesting character in the story. Rupert Grint really shines as Ron Weasley, the Everyman foil to Harry's Chosen One. The Weasley family in general is brilliantly portrayed (by the way, did Bonnie Wright, aka Ginny, grow about three feet taller between movies?).
The other standout actor is Evanna Lynch as Luna Lovegood. Luna could easily have been written off as the Jar-Jar Binks of the series, but Rowling's writing and Lynch's portrayal strikes the balance between the flaky comic relief and a genuinely likeable person. You get the feeling you'd like Luna enough in the real world that you'd be willing to ignore, even embrace her eccentricities to be her friend.
The movie moves along at a fast pace, which is both a strength and a weakness. On the one hand, this long (150 minute) film just about flies by, with great action shots and plenty of adventure. But when things aren't blowing up, it feels like a lot has been left out. If you haven't read the book, you may have to struggle a bit to figure out the whys and hows of some plot developments.
Now, of course, we have to wait for the next installment of the series, which is scheduled for late next year, followed by the final movie in 2011 (Harry Potter and the Deathly Hallows is being split into two movies. First, it's a really big book, second, if you were Warner, would you want to let go of Harry Potter any sooner than absolutely necessary?), which brings me to my one other minor quibble: can we get these kids out of school before the actors start having kids of their own?