The API is nice - but Zillow has a lot of cobranding requirements that has resulted in my cofounder and I creating our own crawler to get the data from Public Record rather than Zillow.
We're not planning for 100% coverage, especially during our beta. But we do plan to add more markets (counties) as we roll out more.
I have yet to find a good alternative - there are a few places that offer up the data already indexed, but they either have limits on the searches per day, or a ToS that does not allow the data to be repackaged commercially. If there was an API out there, even if we had to pay for access, that allowed to search for the data we needed with no cobranding (or less cobranding than Zillow) we'd use it. For now, we'll put up a few more nodes and index the data ourselves.
