First off I have to note that seeing any movie – even if it sucked – on an IMAX theatre spoils you for anything else. Fortunately, Watchmen didn't suck. Not by a longshot. The only thing that sucked was the start time, 3/6/09 at midnight.
From the opening montage showing the (fictional) evolution of costumed crimefighters spanning pre-WWI through the Kennedy assassination up to Nixon's 5th term (yes you read that right), Watchmen grabs you and immerses you in it's lush tapestry of detail, action, violence and story.
Things like the re-enactment of motorcade being shot at in Dallas (and we finally see who the shooter on the grassy knoll was) all the way to the denouement (which was devoid of the book's manmade-telepathic-squid-monster-that-seemingly-teleports-to-New-York-from-another-dimension-uniting-all-world-governments-in-fear) look so good, so real and full of detail and life that you forget you are watching a movie. It feels like you are looking out a big picture window.
Without any spoilers, the plot is that in the US, superheroes (aka masked vigilantes) have been outlawed due to public outcry ("who watches the Watchmen"), so they either give up fighting crime or work for the government. Someone is picking them off one-by-one, and it is up to some that remain in contact to try and figure out what is happening.
Very good mystery/sci fi combo, with also very good characterization. One of the best was the anti-hero Rorschach (named for the Rorschach inkblot test that he wears on his mask. A sociopath/psychopath, the good news (for most of us) is he turns his rage and anger towards criminals. When arrested after being set up, he reminds the other prisoners (some of whom he put there) "I'm not locked up here with you, you're locked up here with me".
There were a lot (40% +/-) of women there, many with other friends and not as couples. Part of this was the love story between the girl known as Silk Spectre and Dr. Manhattan, the glowing blue guy who is the result of a nuclear accident and who is so powerful that he is almost godlike. In fact the US winning the Vietnam war is the direct result of the Viet Cong's deification of him.
This is not a movie for any pre-teens, violence is on the level of the film 300 (also directed by Zack Snyder), sex is sporadic but detailed, and there's a lot of adult topics. All in all a great way to spend an evening sleep deprived.