I've sucessfully used 99designs.com a few times and it's worked out great each time. With each new design need, I go back to the designers that won our prior contests.
i click on some users' folios and i see that even the best of them might have entered 200 contests to have only 5 wins. in short, the only people this is a win for are the founders making off with a cool $40 for each contest started.
I chose to put the contest for the logo, and then work off-line with the winner to do the rest of the design work.
Feedback is key to this process! And definitely check out the designer's profile and his other submissions. It's a good way to see their style and if it matches your tastes. Also a good way to see if they just submit a generic logo and just modify the text for each contest they enter.