And I'll have a full review up later [update: Voilà!], but the short version is this: If you're on the fence about it? Don't bother... It isn't a very good movie whether or not you've read the book; especially frustrating, though, if you have.
The smiley face is a registered trademark in some countries. As an icon of Watchmen, with the bloodstain, it may be trademarked by DC Comics. My frowny version is presented as satire.