I have been using Fiverr for more than a year, spent thousand of dollars to get services from different sellers. So far, of course I have met a few lame sellers delivering below average quality and still able to argue they did the job. Some I managed to get my refund, some didn't.
Nevertheless, there are plenty of sellers whom I repeatedly purchased service from them and they never fail me. So I'll say it's more satisfaction than disappointment, and I will continue to use it.