It is only roughly accurate.
The site uses the official data available to the public. It can't take into account the condition of the house, the condition of the neighborhood, any improvements done that aren't in the public records yet.
If a house near you was a sty and sold way below market value for similar homes then an actual appraiser will know that, zillow doesn't. Both will lower YOUR homes value because of it but zillow will lower it more.
If you have had work done on the house (new kitchen, new bath, additional bedrooms, etc) that don't show up in the official data yet, an appraiser will raise your value, zillow won't know about it. Of course, this assumes the work was permited and done well. Unpermitted and/or badly done work will lower your value below what zillow shows.
at 1:53 PM on Jan. 11, 2009