The Watchmen movie is coming. Don't read "Watchmen."
People are very excited about the Watchmen movie that's opening soon. A lot of people think now is a good time to read the award-winning comic book that the movie is based on. A lot of people are wrong.
Watchmen is one of the most celebrated comic books ever made. It's likely that the film will not be as good as the comic book. Your mind is attuned to change. Thus, if you read a great comic book and then see a decent film, you will be disappointed with the film.
On the other hand, if you see a decent film and then read a great comic book, you're like "even better!" See the suckier thing first.
I read Watchmen years ago, and I'm looking forward to reading it again. After I see the movie.
This is a policy I follow with books too. See the movie first. Rarely the book is worse than the movie. In this case, read the book first, if at all (e.g., The Devil Wears Prada, The Bridges of Madison County, V For Vendetta).
I've heard one good objection to this policy: that when you read the book, you get your own images of what places and characters look like, and if you see the movie first your imagination gets hijacked. This doesn't apply to me, really, because my imagery when I read is pretty minimal, and tend to skim visual description anyway.
However, note that this reasoning does not apply to a comic book, because unlike a novel it comes with its own imagery to hijack you. So see the movie first.