Currently I have 10 homes in various locations... Ohio, Arizona, and Virginia.  I noticed Zillow was advertising to purchase my homes in Arizona for cash.  Note I believe it's only available in some markets.  I thought it was worth a shot so I began communicating with them.  The young man was a pleasure to work with and was very honest and professional, so we moved forward.  

Before the inspection, they offered an amount which was reasonable.  It would have slightly higher than what I would receive if I worked with an agent.  Of course the process would be faster also.  Again I agreed to proceed.

During the inspection a local inspector was present, and a Zillow representative.  Both were taking pictures and documenting their findings.  Both were professional.  The only issue I had is that the inspector left the air conditioning at a low setting of 70 degrees.  In Arizona, that's a big mistake.  Later we went to a local mall and returned in the evening.  We found the house was at 70 degrees!  That's going to show up on my electric bill!  It's not really Zillow's fault as the independent inspector was to blame.  

Also during the inspection, we had several repairs which were in progress.  The outside was being painted, bathroom was being updated, etc.  Because of the repairs, they decided to perform the inspection again to verify everything completed.  

When I received the inspection report and new offer, everything changed!  First they provided an offer which was based on the first inspection, not the second inspection.  I told them the problem and the realtor who gave me the offer ignored what I said.  The Zillow representative stepped in and was professional and said they would stop and investigate the issue.  One day later they said they were firm on the price.  They dropped the offer by $30k and ironically said the air conditioner didn't work.  They wanted $4000 to replace the outside unit.  The reduced price did not match the cost of repairs.  However the inspector took pictures showing temperatures and showed that the AC worked, but later documented that it didn't work.  First we have to understand that I had the thermostat set for 88 degrees (the outside temperature was 110F).  The output from the AC will only drop a percentage of the ambient temperature.  I think they were expecting an output of 50 degrees or so.  I argued with them on the condition of the AC even though pictures showed it was clearly working, but they wouldn't budge on their ruling.

So they thought I would go for it but I didn't.  They thought I wanted to quickly unload the property but I didn't.  

Original value:  $224,000

Zillow Cash Offer: $181,000

I ended up selling with a realtor and getting $224,000, minus closing costs and realtor commission.  In the beginning I was impressed with the service and professionalism, but later everything changed.  They assumed I would accept a low offer and tried to bully me about my air conditioner.  They wanted an air conditioning certification too.  I asked them what was a certification and they couldn't answer the question.  I offered to pay for an AC inspection/certification without paying for the replacement but they only wanted the replacement.

Anyway.  I like Zillow a lot but be aware that investors will be investors.  The home-buying service was a big negative!  

In the future I hope to sell properties on Zillow as FSBO. Hopefully that will work better.

Good luck!