It's bit overpriced, imo. I recently had a chance to demo them at an iRiver store, and here are my quick impressions.
They look...cute. They are very small and portable. A major selling point.
However, the sound isn't worth full $600 tag. In Korea, they can be had for cheaper, so it's worth it, but for $600, it's a rather bad deal.
Why don't you look at Fiio X3 and X5. X5 should have sound quality that is better or on par with AK100 with cheaper price. The downside? It's not released yet. X3 should also be good.
What are you going to use ak100 with? What music file? What headphone? What amp?