A very good film, though I didn't find it as stunning as some people did. I rated it "above average".
For some reason, the beginning didn't seem to work very well--at least for me. In retrospect, the movie reminds me of a car that has trouble starting, at first, then gets rolling smoothly, and finally is racing along at breakneck speeds and performing all sorts of amazing stunts by the end.
I actually gasped and said "oh!" when...
...the convict threw the detonator out the porthole. That, and the other passenger's inability to throw the switch, on the other ferry, was one of the most powerful moments of the film for me--a very forceful affirmation of the basic goodness and decency of ordinary people, and a big collective "Fuck You" to the Joker.
It was as if those passengers were saying: "you may kill us, you twisted freak, but you can't degrade us down to your level, and make us kill each other; we are not like you." They were all tempted--even sorely tempted--but finally stepped back from the brink. It was an unexpectedly positive message for such a dark film. Just brilliant writing and directing, IMO.
That scene, in turn, got me thinking about the person the Joker did succeed in degrading: Harvey Dent.
On the one hand, you could argue that any man could be driven over the edge by such a traumatic experience. But looking back, the movie seems to have hinted that all was not quite right with Dent to begin with. He deceives people with a two-headed coin, and brags about "making his own luck." This seems innocent enough, at first--he uses it to persuade Rachel to go out with him. Their relationship begins with a little white lie--but a lie nonetheless.
Then things take a darker turn, when he uses the coin-flip to torture that one prisoner. We know that he won't pull the trigger--the coin will always come up heads--but his captive doesn't. Both of these incidents hint, at least to me, that Dent is not quite the white knight he's made out to be. The seeds of Two-Face exist inside Dent, long before he is disfigured.
And ultimately, of course, when Rachel is murdered, Batman is wounded as deeply as Dent--but does not go off the deep end the way Dent does. Ultimately, and ironically, the "dark knight" possesses moral resources that the "white knight" does not. He won't even kill the Joker.