Looks like we're entering the Mechanical Turkination of graphic design. At first blush, I think this is awesome, especially for small consultancies/businesses/side projects. Converting a logo to vectors, for example, is something that would probably take me at least an hour and I'd be more than willing to pay someone $15 to take such a task off my hands. It's no replacement for an actual graphic designer, but often you don't need an actual graphic designer.

(The caveat, of course, is that the commoditization of graphic design relies on the assumption that you can treat graphic designers as commodities; if value swings wildly from one of 99design's members to another, then the attractiveness of the service plummets.)