The particular showing I was in was probably about 75% full, but I think four of the shows ended up registered as "sellouts" for the theatre. The mix of people varied in ages. I didn't see as many adults/parents as I would have expected, but there were quite a few people from the local university, and a ton of junior high and high school-aged kids there, and not just from our town. The dress-up was in full effect, even for the older people there. I saw a lot more prop glasses than at previous premieres for this series, and the scarves were everywhere - probably because of the 30 degree temperature drop in the 24-hour period before this film hit the scene.
Additionally, it looks like the film marketers were out in droves before this flick, because there were a ton of new ads up on the walls and in the lobby, including a huge display for Underworld Evolution. Lots of the counter workers as the concession stand were taking one for the team with an assortment of scarves and glasses and faux scars on their foreheads, too.
"Big Kev," who some people might recognize from XM Radio's "Opie and Anthony" crew, also showed up at the theatre, and was going from screen to screen before the show making jokes about how there were problems with this print, and how if it didn't work out, we'd all get to see Zathura in its place. That didn't fly so well, as it seems that the sleepy patrons weren't amused. However, everyone stopped being sleepy when Kev began asking Potter trivia questions for prizes in the room. Too easy, really - if anyone in the room didn't know that Harry could speak parseltongue, I'd most certainly be surprised.
People definitely enjoyed the film, with regular laughter and banter from the audience, and a huge round of applause at beginning and end. What struck me the most was the exit - as I had gotten to the place more than an hour in advance (I just had to play the Fast and the Furious video game, sorry), there weren't too many people there before 11. On the way out, though, it was a wall to wall mess in the parking lot, just like it is when a regularly-timed showing or preview gets out. I probably heard about 15 people talking about how they couldn't believe how many cars there were. And one last thing - if you thought that the costuming was just for the younger kids, think again - I saw one gaggle of teenage females doing the whole Hogwarts-meets-Britney Spears thing with their school uniforms, ties, short skirts, knee-highs and all. Talk about trouble.
In closing, I give it two thumbs up, and a large popcorn for overall amusement throughout the evening. If you're looking for a big review, check out James Rocchi's here.