I had an excellent experience with 'bookdesigner' helping me immensely through my first 99designs competition.
He submitted a great number of different designs and was very responsive with different suggestions and tweaks.
In the end, he came up with a book cover that I think it awesome, and I am extremely happy I decided to spend the money instead of trying to give it a go on my own. He well and truly worked hard and won the prize.
I would be more than happy to recommend his work in the future.