Watchmen is likely to become a cult classic. I just wasted about 20 minutes of my life seeing what the mainstream thought on Yahoo and other sites. While some who haven't read the book loved it, there seems to be a large number of negative reviews. Some are just disgruntled fanboys who cannot accept that a film could never have the rich pacing structure and detail that the novel had. But most are people who simply HATED the movie due to its fringe mentality. They hated the rape scene, dead dogs, dead children, dead pregnant women, a man getting his hands cut off, "graphic" sex scenes (I saw no shots of genetaila? It was only the boobs that made it R-material I thought)...but most of all...they really hated that blue penis.
I have said it being structured in storytelling like a book (though paced like a film) gives it a bit of an uneven pacing and causes it some narrative problems, that is why I gave it an 8.5. However, I now realize Snyder did not just do this to please the fans, but he did this to FRUSTRATE the mainstream movie audience. Those who just came in looking for a simple superhero movie were not ready for a movie that had its action scenes spaced out by 20-30 minutes and unfolded in a convoluted detective story closer to the narrative style of Citizen Kane (though nowhere near as good) as opposed to Spider-Man or X-Men.
BUT
It is a good movie with strong visuals. I think it could live on despite this problem. It will have an "okay" run at the box office that will ensure no sequel (good for us, but perhaps bad for the genre as a whole when it comes to darker comic book movies) and the mainstream will shake its character-driven, extremely violent celluloid off.
And then it will I think achieve some cult status. As I mentioned earlier, there will be those who just get it. It is not as good as the book, but those who are major fans of the book will likely continue to admire the film as a companion piece. But for those who have never read the book or are coming of age to discover some strange unsettling ****, this movie will likely speak to them. This movie will likely live on as a strange superhero movie with a very fanatic cult audience. I'm just guessing here.
I don't think it will redefine the genre or cinema (as the book did for its medium), but it may inspire future filmmakers and a very loyal following well after the studio finishes its triple/quadruple and final dip of the DVD sales.