Choosing a designer is hard. Using 99designs freed me from that task. The final comparison comes down to $300 for a logo vs. $300 plus the time and effort required to find the right designer plus the risk they'll produce something you don't like. I took the former deal, and I continue to think it was the right one.
Dribbble does not change the value proposition I outlined. What rational business owner will choose picking a designer from Dribbble and hoping they'll produce a desirable logo instead of picking a logo from 99designs? To me, 99designs seems like a vastly superior option.