99designs rocks! It's extremely well thought, very efficient and provides competitive price points. I just ran a t-shirt contest and it went so well I did select 3 winning designs. Communication is speedy, the feedback look works perfectly well. Support is available to quickly answer any questions and the range of designers "on hand" is quite impressive. I also really liked to be able to create a poll to help me refine my choices.
I am a 99designs convert and will use it again very soon!