The embargoes only run until a week or so before release - ultimately they want them discussed remember, that's just about trying to focus the publicity into a short period before release.

If the studios think they have a real turkey on their hands (and they'll know this before the critics see it - the focus groups will have told them if they didn't know just by watching themselves) they tend to drop all press screenings.

But you're right that word of mouth will often be more significant than a review. Blockbuster type films tend to be completely critic proof - the reviewers can savage them as much as they like and there's little impact - but if a couple of friend's whose opinions you trust tell you a film sucks there's a good chance you'll steer clear.