Seems pretty accurate for mine.
I've been using Zillow for a good amount of time now....
If you own the place, you can log in and change the home information, that affects the listed estimated value. They obviously can't be perfectly accurate, they don't SEE every house, thats why, to support the website, give it more info!
I changed the info for this house, and the value went up about 20 grand.
I think its an excellent site, and can only get better with time.