I'm really glad I have this in my collection. I almost passed on it when it came out, as I was still coming out of my Marvel Zombie mode, just starting to get interested in a few DC books. I started with #4, and got the back issues before #12 came out. It's along read for a comic book, and was the first I (and likely many) comic readers seriously considered that mainstream comics could tell mature, R rated stories and succeed. Until that point, there were few books that treated superheroes so seriously. There were indies, and sure many of them were overly violent and contained sex, but it was either parody, porn, or just overblown stuff that contained the mature content without much purpose.
Watchmen also made a point of not just telling a story, but working hard to tell it well.