Check out the link below, its a great article about the accuracies/inaccuracies of the zestimate and some of the pit-falls of trying to adhere to "Zestimates" when buying and selling. The error rates in some areas are pretty ridiculous!
Zillow CEO Spencer Rascoff answered that they're "a good starting point" but that nationwide Zestimates have a "median error rate" of about 8%.
Last July, Robert Earl, an agent with Choice Homes Team in the Charlottesville, Va., area, examined selling prices and Zestimates of all 21 homes sold that month in the nearby community of Lake Monticello. On 17 sales Zillow overestimated values, including two houses that sold for 61% below the Zestimate.