I went to go see it at the midnight showing, and I wasn't that impressed at all. It's like they traded a ton of the storyline/dialog for fighting scenes and sex scenes.
Biggest inaccuracies/changes (for those that have read it):!!!SPOILERS SPOILERS SPOILERS!!!-All of the street scenes were taken out. (The news stand, the 2 Bernards, the Palehorse concert, Joey and her girlfriend, etc.) The news stand is only showed in the background once.
-The bit with Hollis Mason's death is removed
-The sex scene with Laurie and Dan is lengthened greatly
-Violence, blood, fighting, curse words, etc is intensified greatly
-The part where Jon and Laurie debate about Earth is shortened (the perfume bottle thing isn't included either).
- The scenes with Dr. Malcolm are shortened greatly (the sub-plot with him and his wife is ignored), as are the flashbacks.
-Never explains how Rorschach got his mask, or explained the story with Kitty Genovese
-Never completely explains what the "Keene Act" is.
-Never completely explains Veidt's Antarctic retreat, or Bubastis.
-Dr. Manhattan zaps Laurie's brain in order for her to remember, instead of her remembering on her own (about the Comedian being her dad).
-The scenes with the detectives are taken out (except at the very beginning and when they capture Rorschach)
-The scenes at the New Frontiersman office are taken out (except at the very end, when he drops ketchup on his shirt)
-The explosion in New York is blamed on Dr. Manhattan, instead of the giant squid/alien thing.
-Rorschach kills the serial killer with a butchers knife to the head, instead of burning him alive (from the part when he "becomes Rorschach").
-The name of the group is switched from "Crimebusters" to "Watchmen" so that people who haven't read it won't walk out of the theater confused about the title.
-Captain Metropolis is taken out of the Crimebusters meeting.
-The whole aspect of "the island" is taken out.
-Dan attacks Veidt in the end, instead of going to sleep with Laurie by the pool.
-Some of the parts with Rorschach's journal were changed (though most were pretty accurate).
SPOILERS OFF
Maybe I'm just being a Negative Nancy though.
Most of the people coming out of the theater seemed to be talking about it positively. My friend said she'd see it again, while I wouldn't even consider buying it on DVD.