In my experience it is woefully inaccurate, at least from a pricing/value perspective. I remember looking at it when I was shopping, and the values they gave were just about useless.
I just checked again, out of curiosity, and it still shows some funky prices. Their values don't seem to tie to much of anything. That said, they seem to think my house is one of the most valuable in the neighborhood, so who am I to argue?