I don't mean to make it sound like all critics are right, but generally, if critics are lauding a film, its usually pretty good (and if they're all saying its bad, its probably bad, w/ some exceptions).
What I'd like to understand is, if you DO feel that critics are idiots and don't know anything, how else do you determine whether a movie is worth seeing? It seems that most of the people who are seeing movies on opening weekend won't know anyone who's seen it, so that's not the main reason. I somehow doubt people in this boat read rottentomatoes or other internet hype sites (ain't it cool, etc.). So basically, what else are you going on than the preview, which was simply made to make you want to see the movie, be damned if its actually any good (again, given that some movies are made around the premise that will make for a good preview, not necessarily a good movie).
As for the bad critic who said mystic river was better and more memorable than return of the king, would you like to compare notes on how many movies have hyped themselves to be the best thing since star wars? There are morons everywhere, the question is, who's more likely to be a moron? Surprisingly, I'd say the movie studios who make the previews would be more likely to be "wrong" about the quality of a movie than a critic or some other outside source.