I've used 99designs a couple of times to source designs for ebook covers. The creativity of designers is sometimes hit and miss, but it's always fantastic to get a wide range of designs, and it's great to be surprised when a creative designer comes up with something fantastic that I hadn't even thought possible. I plan to use 99 designs for all my future projects. Ideally, one has a good working relationship with a fantastic and creative designer that understands your needs - but, that's sometimes hard to find. When you don't have someone like that in your network, 99designs fills the role spectacularly.