99designs is awesome. I use graphic designers all the time and I was very concerned about the quality I would get on 99designs: I didn't want to waste my time.
I was really surprised. The quality of the designs I got was TOP NOTCH, and the designers were really easy to work with. I tried to comment on every design that came in, and the designers responded with new designs according to my comments.
Aside from the incredible choice and talent I got: the turn around time was lightening fast. We went through over 80 versions in just a couple of days. In less than a week, we had our final files and we could move on with our project -- try getting your designs done that fast with a single design shop: you won't.