(UPDATE: I was thinking about it this evening, and realized that almost my entire immediate family went to this movie within the first 24 hours, with all but one person going to the midnight show. Sweet Little Sister has actually already been twice. There's a bit of the magic of Harry Potter for you--I can't think of anything else that we all have so resoundingly in common!)
(UPDATE 2: Just got off the phone with Nice Mom--who loves the movies, but stubbornly refuses to read the books--who said she had no trouble following the movie at all. Maybe I'm underestimating people....although she was a little confused about why Harry and Cho didn't eventually make up.)
(UPDATE 3: Last one, I promise! I was also thinking tonight, unbelievable as it might be, this is the first time I've gone to the midnight show of a Harry Potter movie. I don't know why I didn't go for Stone or Chamber, for Azkaban I was actually on my honeymoon, and for Goblet I opted to wait so I could see it on IMAX. I have to say, at 11:45 last night, I was starting to get sleepy and wonder whether or not the choice had been a good idea. Once the movie started, however, I woke up quickly. And, it was a lot of fun to watch the movie with so many other big fans--there was lots of spontaneous cheering, clapping, ohhing and ahhing that I hadn't ever really experienced before. I think it was definitely worth the lost sleep.)
Overall, it was great. I liked the darkness, I thought they translated the sets well, I thought the actors were good, and I loved the soundtrack. The funny parts were very funny, there were several parts that were touching, and although I didn't love it-love it, I think it will grow on me. It's probably about my second or third favorite of the Potter movies (after Goblet, and maybe tied with Azkaban).
In the event you want to be surprised by the movie, I'd stop here, but for the rest of my thoughts keep reading....
The older actors were great. Umbridge was incredible. They actually made her worse than she was in the book; she tortures the members of the D.A. en mass, and even some really little kids. She's without a doubt the best villain yet, and I understand why reviewers are already saying she might be the Potter series first Oscar winner. Alan Rickman only had maybe ten lines, but each syllable was perfectly Snape. ("Obviously.") Even Mr. Gambon wasn't as bad as usual. After Trewlawny is sacked he snarlingly asks a group of students if they "shouldn't be studying", but other than that he didn't butcher Dumbledore too badly at least.
And the kids were good too. Daniel did well with Harry. I think I liked him best in the DA scenes, he really was able to come across as a competent leader. Rupert and Emma were much more subtle in this one, in a very good way. They let Ron's character grow up a bit--no silly, scared faces--and Hermione's outbursts were much more mature. Ginny was always in the background--occasionally giving Cho and Harry jealous looks--and I liked it that they really showed that she's becoming a super-powerful witch. Oh yes, and Evanna Lynch--her voice, her delivery, her hair, her clothes--I'm not sure if she's just that way, or if she's just that good of an actress, but in any event she was Luna Lovegood.
I also generally liked the way they portrayed a lot of Harry's inner torment. So much of this book happens in Harry's head I could see where it would be hard to translate, but by using flashbacks and the like I think they did a credible job.
My problems with it?
Well, for starters, although as I said, the parts that were funny or thrilling or what-have-you were all very much so--but the parts that were scary or sad didn't match. In other words, the highs were very high, but the lows didn't sink very low. So that you understand my perspective, I still cry (like a baby) every single time I watch the end of Goblet of Fire--it's actually a little pathetic at this point. But when Sirius died in this movie--even though the movie got all slow-mo and Harry screamed "Nooooo" as Lupin held him back--it was more kind of like, "Oh....well....hmmm....there goes Siruis...."
Second, although I always kind of have trouble really being able to tell (because I've read the books about a million times) I think that if you haven't read the books, at the least you'll miss out on some of the more subtle plot twists and at the worst you might end up pretty confused. Again, so you don't mistake where I'm coming from--I know the books and the movies are separate entities and I like it that way. (I think my least favorite Potter movie is Sorcerer's Stone, which was pretty much a direct translation--there was nothing in it that I hadn't already pictured in my head which made it kind of boring.) But for this movie I think just 5 or 10 minutes more explanation--especially at the end--would have cleared up so much without making the movie unduly long.
For example, they never explain that it's Snape who alerts the Order that Harry's gone to rescue Siruis. They never explain that Siruis is also known as Padfoot, but then Harry calls him Padfoot--just one extra line would have completely cleared that up. During the occlumency scene Harry sees the memory where his father torments Snape, but then nothing really comes of it--he just kind of leaves and forgets that he saw his dad being an ass.
Further, Dumbledore never properly explains about the prophecy--i.e. that he knew what it said and how he knew what it said. They never really even explain that it was Professor Trewlawny that gave it. Also, although I know he could have done some growing-up between the last time we saw him in POA and this movie, it's never explained how or why Dudley Dursley went from a dumpy, little kid to huge, hulking thug. When the Advance Guard picks Harry up at Privet Drive, Lupin isn't there. So, Harry goes flying off into the night with Moody (who he last saw as a bad guy) and a bunch of people he doesn't know. Ohhh, and I almost forgot--when they got to Grimmauld Place, instead of showing Harry the note and explaining about the Fidelius Charm, Moody just taps his staff on the ground and the house appears. Tonks is just kind of there--you never really get anything about who she is or why she's changing her nose into different shapes.
I could go on and on, but bottom line, if you didn't already know what was happening I think this all could have been really confusing. The movie just doesn't really stand alone all that well.
Next, there were a couple of things that set up potential continuity flaws later. For example, instead of having Marietta give the D.A. away, they had Cho do it. For awhile everyone thinks she did it for some unexplained reason, but eventually it comes out that she was under the influence of veritaserum. This worked as a great way to both out the D.A. and break up Harry and Cho (the whole Valentine's Day scene in Hogsmede was omitted). But I think that creates problems for the next movie--if Harry forgives Cho for giving them away (because she obviously couldn't help it) then why don't they get back together?
Another thing I just don't think they interpreted well was the fight between Dumbledore and Voldemort. In the book Dumbledore seems very cool and collected when dueling Voldey, in the movie he seems desperate and scared.
My last problem with the movie was that it kept hitting me over the head with these big, grand sweeping morals. This is something J.K. Rowling has said she tries not to do with the books, which I think made it particularly worse. I understood that the theme of the movie was that Harry wasn't alone, he had friends and family that cared about him, etc., but I think most people would get that without the characters constantly, verbally reiterating it. It was just a little overkill.
Despite my issues with it, overall I did like the movie ....I may even let myself get talked into going to see it again.