I am familiar Zillow and quite perplexed by their "Zestimates." I live in the midwest where real estate valuations continue to decline. I subscribe to their Zestimate updates that provide monthly updates on the "value" of my properties. I have noticed that valuations will change quite significantly from month to month, sometimes favorably and other times unfavorably. In the current market condition, I know that my valuation is not going up, now or anytime soon. At the end of the day, their Zestimates fail to consider the quality/condition of the property. I think it is mostly based on historical activity. Most of the sales these days that are foreclosures will dramatically reduce the averages, however many of these properties are not even in what I consider livable conditions (again, I am only speaking of the midwest). I think the site is intriguing and certainly fun to play around on. However, I would not put any stock in anything on Zillow as the source of their information is uncertain and there are obvious flaws in the Zestimate system.
I am interested to hear what everyone else thinks of it, especially realtors and investors!