Well, any poll begins with a bias. Merely the order in which two names are listed will create a bias. Many Freepers blew off the polls in 08 and 2012, and the polls were right. On the other hand, the pollsters universally missed the 2014 midterm results, the Israeli election, and now the Brit election.
They can be pretty accurate when they self police. When they don't adjust for their own biases, they are not.