This Montclair house, 7 Warren Place, (left) is on the market for $769,000. For its reval worth, you can find it here, on the Appraisal Systems site, in Neighborhood 7, for $740,000. Or you could take Zillow's zestimate above, which predicts a sale price of about $639,014.
So who do you believe? Realtor, appraiser or Zillow. Let's watch this one and see what it ultimately sells for. Tell us how your home stacks up.
Meanwhile, here's a much larger price discrepancy...
This is 429 Orange Road. It's on the market for $1,300,000. Below, Zillow zestimates a much lower sale price.
Then there's the reval -- it comes in at...
Taxes are listed as $13,400, but the property, circa 1852, is on .95 acre. Hmmm...

Zillow's estimator formula uses past sales of the home (if relevent data exists), plus sales data of comparable homes in the neighborhood (if relevent data exists). It is not meant to replace a human appraiser that will come in and do an inspection and form a custom value specific to the home and timing of the marketplace.
Zillow is better suited to more stable markets in other sections of the country, not NJ. At the very least, its good for identifying exact addresses for homes you'd like to buy ...