Yeah, I just gave a superhero movie five stars. The thing is, compared to other movies in this genre, this is a masterpiece. Superhero movies are usually very simple and bipolar: here's the bad guy who will die, and here's the good guy who will discover his powers and then save the world with them. Not so in Watchmen. Moral ambiguity is part of every aspect of the story, leaving you to decide who is good and who is bad. This is amplified by the fact that only one character actually has superpowers.
In addition, the cinematography, special effects and general production values are off the charts. Much like 300 before it, this movie oozes coolness and badassery, except in a much more calculated fashion this time around. The acting, while not oscar-worthy, is exactly what was required for each character. Jackie Earle Haley's portrayal of Rorschach, in particular, is absolute perfection.
Now, I'd read the comic before seeing this, and as with any adaptation, I was afraid that it wouldn't be up to par with the original. Surprisingly, I vastly preferred the movie to the graphic novel. Except for some very minor elements that aren't introduced properly (Adrian Veidt's pet, for example), this is the most faithful adaptation I've seen so far, actually going as far as improving the original.
Though you should be mindful that this is a very explicit and gory movie, it represents the very best comic book movies have to offer. However, it is not for everyone, and serves better as a companion to the comic.