Questions of whether or not a movie is "overrated" or "underrated" is difficult, because it first presupposes so much about how others have rated it. Tyler Cowen just linked to one list of underrated science fiction movies, most of which I cannot comment on because I haven't seen. However, I feel that I must ask, where are the metrics?
There seem to be two types of "underrated": movies that have not been watched enough, and movies that have been watched but not fully appreciated enough. Both of these methods could be measured using imdb. If a movie has a high score but few views, it has not been watched enough. And if you gave a movie a high score (or some other group did, say the Top 1000 voters), but others did not, then the movie would be underappreciated.
Determining if a movie has not been watched enough is more complicated (you'd have to define an average), while determining if a movie is underappreciated is more subjective (who says that your opinion is the end all and be all?). But both are doable empirically; look out for it in a Tuesday Statisticz coming to a blog near you.