I was working with an agent and I told him I saw several houses I wanted to look at on Zillow. Sent him the list and all but one was sold. He said the listings were in some cases as much as 6 months old.
After that, he just plugged me into the MLS listings directly and then every time a new house would show up, I'd know about it and could email him if I wanted to see it or not.
I totally agree about not trusting 3rd party apps. MLS, unfortunately is the only reliable source I know of.
To amplify: I don't think it's any accident that the 2005 housing fiasco happened in this industry. Terrible pricing information (gaps + timeliness), as well as terrible conflicts of interest (buyers brokers are incentivitized to make the buyer pay as much as possible) make me wonder how much longer these dinosaurs are going to continue to operate the same way.
A loose analogy: if you buy stock with 100:1 leverage, and the stock price declines 10%, you lose 1000% of your original investment. The big problem here wasn't really the 10% decline in stock values (significant but not catastrophic), rather the fact that you had 100:1 leveraged exposure.
As long as the regulations are in place that protect them from market competition and innovation.
In short, trust none of them, use multiple services, and keep looking constnatly. I found my house through my agents MLS system, which was about the time it showed up on the aggregates.