... and I'll have a full review up later (update:here it is), but the short version is this: If you're on the fence about it, don't bother. It's not a very good movie whether or not you've read the book.
The smiley face is a registered trademark in some countries. As an icon of Watchmen, with the bloodstain, it may be trademarked to DC Comics. My frowny version is presented as satire.