Well, the only caveat is that the system doesn’t take into account of the interior, so it’s only a rough estimation. But it does give a valuation all the houses in the neighborhood.
This is a website started by former Microsoft employees who were curious to see how much Bill Gates’ house and Paul Allen’s house are worth.  I believe their system is based on comps as well as statistics based models.
I did a few searches of properties in my area and the results were pretty close (within 10%) to what market rates are in the area. It’s a good tool just to quickly get a rough estimate of a property. Give it a shot!