How accurate are web sites that determine the value of homes?

How accurate are web sites that determine the value of homes?

Chad Lund (Real Estate Agent) gives expert video advice on: How do I set a price for my house in a slow market?; How do I know if I'm asking too much for my house?; What is a 'bottom line'? and more...

Websites for price comparisons are not always accurate. They are taking in tens of thousands of homes, not comparing the exact upgrades of a home. So, I'm not real satisfied with where web sites prices come in.