Crowd-sourced testing

Imagine 40 people from around the world suddenly scouring your site for bugs,and reporting them to a central system that you monitor and approve / reject. That was my experience recently with uTest, who provide the platform and the community for this crowd-sourced QA service. uTest claim it covers several types of testing, from functional to usability, but how does it compare to traditional usability testing and quality assurance (QA)?