I saw it at midnight on Thursday and the theater was packed. They sold out eight theaters and everyone got there an hour early. Heath Ledger owned that movie, and he deserves an Oscar. Otherwise, I enjoyed the movie and thought it was pretty good, but not four stars good. It's not worth all the hype and the fuss the critics are making over it. And the plot was better than Batman Begins.
Thursday's midnight showings here were sold out at 4 500+ person screens in a theater down the street, 2 500+ screens at another theater, 2 500+ screens at another, and one 300 screen at the theater I ended up seeing at -at it's second 200+ screen. I bought my ticket at around 10, got to the theater at 10:30 or so and there was a small handful of people there.
I went and saw this afternoon with my roomate who bought our tickets this afternoon for what turned out to be a sold-out 3:30p showing -very unusual for this theater.
Now, granted, we live in a very saturated theater market. There's scores of screens in reasonable driving distance of me. So getting a ticket isn't an issue for me. I can't see it being all that different in other places. If you want in you can get in without over-paying for the ticket.