I think this season has given the casual fans of the game exposure to Paul Pierce. They probably didn't get the chance to see him play very much in past years, because the Celtics hardly got any national TV time for a few years running.
Perhaps, group think may have shaped them to believe he was overrated, because of the records his teams have had over the years (before this year, a couple were good, some were bad, some were borderline).
I don't think he is "underrated" anymore, but he is definitely not overrated. History will remember him kindly as a winner from now until eternity, no matter how poorly he may fair in the future, because he's won the chip at least once, and has been Finals MVP. Those things alone will guarantee him a spot in the hall of fame - and deservedly so.