"Picture a walkable neighborhood. You lose weight each time you walk to the grocery store. You stroll home from last call without waiting for a cab. You spend less money on your car—or you don't own a car. When you shop, you support your local economy. You talk to your neighbors."
WalkScore uses Google Maps API, so you can check your area only if Google Maps includes businesses from that region. The score is just an estimation and doesn't take into account issues like climate or crime risk. Nunovo thinks that "the walkability of a place shouldn't be based on whether there are businesses from Google's directory within a mile. Rather, walkability is determined by qualitative factors like the width of pavements, traffic noise, trees and views that combine to give a place pleasurable and interesting qualities." Even the site admits that "you should use the Web 3.0 app called going outside and investigating the world for yourself".
The site might be useful if you want to buy a house, to find a quiet neighborhood or to get a listing of the most important local businesses near you.