I just returned from a midnight showing - and am still digesting - so will keep my review short.
Pretty great movie. The Joker character, by large virtue of Ledger's portrayal, was scary, funny and a sympathetic character. By far the highlight of the movie. I thought Aaron Eckhart did a pretty decent job with Harvey Dent. Christian Bale was fine - but his Batman "growl" was a bit much. Movie was quite long - but haven't decided if unecessarily so.
An aside, not related to the movie:
I saw, like I said, the midnight showing (actually, they had 12:01, 12:02, 12:03 showings). 14 screens in one theater - 6 lines, all huge (I showed up at 10:30 - met a couple of friends there since 9:30). People in all manner of costumes. I may have been near the oldest in any line in my mid 30's.
My line was the last show. It collapsed into a literal sprint of 800 people for the theatre - complete breakdown of order. Show started at 12:45 (previews).It was pandemonium. There were kids as young as 10 in this show, which got out at 3:35am - they seemed unescorted. Insane stuff.
There were also 3am and 6am showings around - all sold out through the weekend. This movie is going to make unheard amounts of money.