From the customer's point of view, this is an absolutely wonderful concept - you couldn't possibly get so many ideas from one single designer in such a short space of time. The wealth of ideas helps you to refine your own ideas, too.
Loved the ability to poll friends and experts - in the case of book covers, it's so important to know what kind of cover will attract readers to your work.
Customer service is excellent - I had several questions along the way, and all were answered promptly and to my complete satisfaction.
We plan on using 99designs for all our book covers (www.shelfstealers.com.)