I confess it took me a second viewing to start appreciating a lot of things in this film because I had not (and have not as yet) read the original graphic novel it originates from. The first time I saw it I definitely did not pay proper attention......but I decided to give it another go one night.
Second time around I found it totally absorbing. Although the primary story is the investigation into the murder of The Comedian - one of a group of masked 'superheroes' who once protected, even though only one ever had real abilities - by the vigilante Rorschach and the huge conspiracy that is uncovered, there is a multitude of other themes and propositions to be discovered, as so many other reviewers have well expressed in their reviews.
It's something which genuinely improves with multiple viewings. Yes, it is pretty long but at home on DVD this hardly matters. The alternate 1985 realised in the film is stunning and there are countless details present that add to the realism, not least visuals that are spellbindingly put together with music that seems odd at times, but more than not complements the visuals well.
I don't think you necessarily need to have read the novel to appreciate this film as a piece of cinema on it's own (although I can only imagine how much more it adds having read it) but it is a film that you need to devote proper time to. In years to come, I think this will age just nicely!