Zillow was started by some ex-Google employees. The basic coding is to pull information from county records and local MLS boards to come up with valuations. In general, it's a great tool, however, the mass data collection process often delivers incorrect, or incomplete data. If you read the Zillow discussion boards you will see endless complaints of house values being under or over reported. Another issue is homeowners posting their house for sale using the 'Make Me Move' tool with an inflated price. Often, sellers think that's an actual market value.
Brokers and lenders never use Zillow to determine values. They use actual sold prices and make adjustments for property conditions, loan concessions and neighborhood issues. That's why lenders charge $500- for an appraisal - it's a lot of work to get it right.
Currently, there is a big battle between many of the local MLS boards and Zillow. The boards are not fully reporting sales to Zillow and their data is becoming less accurate. There are some legal threats and in general lots of confusion.
So, I personally like Zillow and use it occasionally for quick data hunts. However, these days it does NOT have all homes for sale listed on it. Just like the MLS doesn't have every home for sale listed. I recommend when using Zillow to remember that it's a broad, and not fully accurate, look at total market conditions.
Zillow sales people calls me frequently trying to get me to sign up for a zip code. If I pay them approximately $500- per month, regardless of my qualifications and experience, they will feature me as one of their recommended brokers. I've declined their offers.
Currently, I have a 5 Star rating on Zillow from the few customers that have bothered to report their experiences with me.