Noticing that there was no Zillow app for Android, I recently embarked on writing one. Pretty simple, assuming they make their data available in a reasonable manner.
Taking a look at their terms of use made my heart sink a little. Quite complicated, and mostly targeted towards integration with web pages. I sent them an email asking whether an Android app would violate their terms of use. A few days later, I received a response. Yes, an Android would violate. They didn’t say exactly how, or why.
Here’s a tip for API providers: it’s a good thing when third parties offer to integrate with your API, creating free applications that make money for you, with no personal gain to them.
Regardless, as it turns out, the API itself wasn’t satisfactory (by design I think). If you go to the Zillow site and search, you get very rich results. The API doesn’t return the same data. Consider the most basic use case: given an address, show surrounding Zillow data. The API can’t handle this. All it can really do is take in an address, and return specific results. And, the address needs to be very particular. It’s not good about handling fuzzily-entered addresses. The API docs say that it returns data for surrounding addresses. It may return one or two, but that’s it. As I mentioned, the same search on the Zillow site returns tens of results.
Zero Credibility 
I was complaining to myself about the same thing. The addresses have to be correct to the letter and sometimes won’t even yield a result if they are 100% correct. The same search on their site provides far more and richer results than the API. I wish the API would at least return whether the house was for sale, sold, in foreclosure, pending, etc. It makes it hard to build an app to find good buys in the market based on comps or recent sales if I don’t know what the status of the sale is.