For a single data point they are useless. But I thing a rating in aggregate could be informative. Of course, the rating is self defeating as too many people who were positively surprised will raise the expectations.

For movies I would love to have reviews in style: Somebody writes up their expectations before going to the movie, and then rates the movie according to that. I find most "press and critics" reviews useless as if somebody is not a fan of story-less action movies, then why the hell is he reviewing them.