As long as there are already many anecdotes, I'll just add mine (I've always taken the zestimate as Zillow's working model that probably needs lots of tweaking; it's not like the MLS's are eager to give them data).

Anecdote: My folks live in a golf course neighborhood where their house is not on the course, and it's smaller than the rest. However, my folks house is valued based on these extravagant golf-course side properties that are actually quite a bit more $/sqft. Zillow needs to have a human go through manually and adjust for these subtle differences, IMO.

Despite dome shortcommings, Zillow is a godsend compared to the systems local MLS's use, just the listing dates/listing removed dates/ last sold for/ days on market dates are worth a lot. Realtors can give you this info, but it is not nearly as frictionless as it could be.

Zillow is still looking for that disruptive go-to-market strategy (they already have most of the tools). But, the Realtors aren't going to give up their ~$6K per transaction fees easily. They're much more connected than cab drivers.