These sites are "review aggregators" -- they take many different reviews from across the web, assign them scores, then produce a kind of super-review. I think these can be useful for a quick temperature reading (so you can avoid, say, The Love Guru), but not everyone agrees with the practice. Nicholas Deleon over at Crunchgear, for example, thinks these sites are "dumb" when they apply this thinking to video game reviews:
I dislike the practice of aggregating review scores, be it on Metacritic, Game Rankings, Rotten Tomatoes, etc. You can't quantify opinion to begin with, assigning a numerical value to how you feel about a game--if you can tell me the difference between an 9.0 and a 9.1 you deserve a biscuit--but then to average several opinions together and wind up with a nice, neat "metascore" is absurd.
As the Oscars approach, we plan to take a look at the effect of review aggregators -- how they've changed the ways we make decisions about our entertainment dollars, how they impact our expectations of films, and what professional movie reviewers think of them.
What about you? How do you decide what movie to see at your local theatre? Which do you trust more -- aggregate film scores or individual reviews?