I don't know how accurate these things are, though my gut tells me not to trust them. That said, as a recent home-buyer, I can state without uncertainty that real estate agents hate Zillow, and Zestimates in particular.
Status Quo in my experience goes:
1. List your house (usually at slightly above whatever the realtor thinks it might sell at)
2. Someone's willing to buy at listing price +/- 5%
3. Bank sends an appraiser whose job it is to figure out what a house is worth (guess what: see #2 for relevant demand evidence), and magically the number comes remarkably close to the price sorted out in #2 massively most of the time
I've never heard a real estate agent complain that zillow's numbers are too low, mind you... only ever that they're too high, which makes sense to be frustrated by if your biggest challenge is controlling your time-costs.
Only seemingly unrelated, just in case, the classic from Frakonomics: