I liked it, I didn't love it. It looked quite pretty, and had a good visual style. Not as good as 300 or Sin City, but a much harder subject matter, to translate the cheesy 4 colors of the original Alan Moore comic. It was a bit heavy-handed in its 80's references and its symbolism and its use of 80's music (which was a bit too obvious, a bit too loud; the nytimes was especially bemused by its use of 99 luft ballons. Even the normally background-y minimalist music it used called attention to itself given that it was a famous Philip Glass piece). I was especially amused by its blatant use putting the World Trade Center in the background of several scenes, with the express purpose of building the mood of unease that pervades the original book (albeit anachronistically, given the symbolism of the towers has changed much since 1985; also interesting contrast to spider man II which went out of its way to go back and digitally scrub all images of the World Trade Center from its movie).
As others all say, I agree, the opening montage was nice, and nice fake cameos of minor 80's celebs, like Annie Leibowitz, who serve as powerful signifiers. And as others have said, Rorsharch was especially well acted, he was the one part of the ending that moved me. And yes, the silk spectre-nite owl relationship was painful.
As for the new ending that was much talked about, it really maintained the same flavor as the original. Which is important because the ending is the heart of the movie. I also thought the ending was appropriate. a tidier way to do it, without introducing the deus ex machina of aliens. Though the ending fell flat for me. Perhaps because it is the kind of ending that once you know it, doesn't work for you anymore--you lose the oh shit-ness of it. Or perhaps because the movie dragged, or perhaps because it was the one part of the movie that didn't copy its dialogue from Alan Moore.
AO Scott called the ending juvenile, not just this movie, but the book as well. I remember being blown away by the concept when i was young. But maybe it is juvenile. I don't think so. But it forces me to reconsider. Because while it packs a helluva a bunch as narrative, it doesn't hold together upon further reflection. Human nature is not as simple or easily quelled as Moore implies.
This movie also made me appreciate the connection with Batman. Rorsharch and Nite Owl are two reflections of Batman, Nite Owl for his gadgets, Rorsharch for his Nietzchean upermensch sense of justice. This was highlighted by the use of the original 1980's burton batman theme song, and with the scene of Rorsharch enjoying the antagonism of his fellow inmates, echoing the scene in Batman Begins.
With time, my esteem of the movie has gone up. Reading the original again, I am amused at how close he stays to the original book. Probably the most faithful adaptation of any comic book. But it mostly works. Still, you walk away feeling you are missing impact, if that can be fixed, perhaps with more gore, perhaps with tighter writing, perhaps with tighter editing of the fatuous love scenes that slow the film down.
Final Grade: B+