I'm not exactly sure why that is. It may be because the show was amazingly close to the tone of the book, overall, as well as to the events. I felt like I'd been watching the show, not reading. Unfortunately, while I really liked Anna Paquin's Sookie and Sam Trammell's Sam, I didn't care for Bill at all, and that carried over to the book. I can't tell if I'd feel any differently if I hadn't watched the show first.
So I can't say I liked one better than the other (book versus movie), which is a rarity.
It's always more straightforward when things go the other way, and I read the book before seeing the movie.
A true Harry Potter fan has to have certain expectations going into a Harry Potter movie:
1. The casting will be mostly great.
2. The atmosphere will be awesome (kudos to cinematography, set dressing, props, costuming, makeup, etc.).
3. The quality of the acting will have improved (for the young ones, anyway--the older ones are mostly awesome, too).
4. The script will suck.
We all understand--intellectually, anyway--the difficulties inherent in condensing these very chock-full books into a 2-and-a-half-hour film. But some things are just incomprehensible to us. As a writer, I get putting your own stamp, your own twist, on an adaptation. But you don't f*** with Harry Potter.
For the most part, Half-Blood Prince is very well done. I found it to be the best of the six movies, overall. I was so pleased with the smooth editing, a huge improvement over the jerking around of Order of the Phoenix. I also--after the initial moment of disapproval--liked the way they incorporated Draco's thread into the film. It couldn't have been handled the way it was in the book, it just wouldn't have worked. And to see his desperation, his intensity, was a treat. Tom Felton did a fabulous job.
There was a ton of humor in this film, too, more than most. Ron and Harry under the influence of their various potions were delightful breaks of normal character. (One fave moment was "and the pincers" with the little hand gestures and clicking.) Through the bulk of the film, I was okay with how they'd "outed" the Harry/Ginny feelings for each other and the Hermione feelings for Ron, all of which were much more obvious than in the book. Since the movie gives us more than one point of view, that was realistic and fun. And it helped solidify Harry and Hermione's sibling-type friendship, which will hold them in good stead for the final movie(s).
But it all kind of fell apart at the end. There were some elements that worked, but they were among others that, for me, didn't work at all, and left me dissatisfied.
1. That kiss. In the book, Ginny gives Harry a hard, blazing look and runs to him, throwing her arms around him, and he kisses her deeply. It's TOTALLY a movie kiss. In the movie, it's gentle, he doesn't know it's coming...and they completely drop the entire thing until the very end, capping it with a tossaway "Ron's okay with it." With what?! There's nothing going on!
So okay, big deal, at least they're together, right? But then...
2. The kiss happens while Ginny is guiding Harry to hide the potions book in the room of requirement. Grrr, because only Harry has been in there, alone, when it's a hiding warehouse, but never mind. And the changing of the reason he's hiding it cuts a lot of pages and it's believable, it fits the movie story, so okay. But she has him close his eyes so he doesn't even know where it is. This shreds any hope I have for the final movie(s). (In the book, Harry marks the book's location with a bust, wig, and tarnished tiara that turns out to be the diadem of Ravenclaw, one of Voldemort's horcruxes. Without that, there's no reason to return to Hogwarts, where the epic final battle takes place.)
And hey, while we're on the topic of horcruxes...
3. They dropped a lot of the Dumbledore/Harry discussions and exposition. Sure, it would be slightly tedious and drag out the movie, but please. Take five minutes to lay out how many horcruxes there are left and what they are likely to be, so Harry's not working completely blind. (On the other hand, nice little touch with the ring and the flash and Dumbledore's dawning expression as he realizes what it means.)
Then we get to the very end. The inciting event that puts Harry on his path to death and Voldemort's defeat. Aaaaannnnndddd...WTF?
4. Why not have Dumbledore freeze Harry under the cloak, as in the book? I found it impossible to suspend disbelief that Harry would stand by like that, or that Draco could disarm Dumbledore without the distraction he had in the book. Snape knowing Harry was there feels wrong, though I don't think that change has a huge impact. I understand why they dropped the battle scene, though I lament the loss of the connection with the other DA members. They don't want to detract from the last movie(s), and it would have been expensive and add length they didn't want to add (though I don't think a three-hour movie would be a problem; it never is!).
5. Harry's confrontation with Snape left me wanting, too. Some of it was spot on, like "Fight back, you coward!" and "Don't you call me coward!" But the "Until you learn to keep your mouth closed and your mind shut..." line was highly significant. And I was completely distracted by Hagrid's hut being aflame and Harry not at all concerned that Hagrid might be burning alive. (In the book, Hagrid is outside after the Deatheaters come past, and it's Fang inside, with Hagrid battling.)
The end result is a strange mix. Every other HP movie, I've had to see multiple times. The first time is to get through the "they changed that...they changed that..." compulsion. The second time is usually a lot better than the first, and I often see them a third time, just to absorb the whole thing or catch details I missed.
This time, I feel more satisfied with the movie as a whole. It engaged me fully, so I wasn't muttering mentally all the way through. Yet I'm unhappy enough that I don't want to see it again and feel even more dissatisfied.
The one thing that hasn't changed, though, is that it made me want to read the books again, despite having just done so a couple of months ago. :)