If you're looking for a logo or page that will fit in with your existing look and feel, those sites are pretty good. But for me personally, I got a lot more traction by being able to sit with the designer and really give them a sense for what I'm trying to accomplish. So, it really depends on what you need.

Because you are asking the designer to do all the work without the guarantee of getting paid. The result is that you get a crappy product, where as if you just bit the bullet and hired a designer you would get something that you could actually use.