Check out the LA Times coverage of Zestimates and their median error rates.
Inaccurate Zillow ‘Zestimates’ a source of conflict over home prices
[…]in New York County — Manhattan — the median valuation error rate is 19.9%. In Brooklyn, it’s 12.9%. In Somerset County, Md., the rate is an astounding 42%. In some rural counties in California, error rates range as high as 26%. In San Francisco it’s 11.6%. With a median home value of $1,000,800 in San Francisco, according to Zillow estimates as of December, a median error rate at this level translates into a price disparity of $116,093.
To Read More: http://www.latimes.com/business/realestate/la-fi-harney-20150208-story.html