I have had one of these for a couple of weeks now. A very nice piece of kit- quite sturdy, well-built, aesthetically pleasing, etc. I am using it to feed an SRM-717 going to my 009s. I changed from my previous DAC (Fostex HP-7) primarily because of the balanced outputs and its jack of all trades list of features. I have it plugged into a Mac Mini using USB and reading lossless files off of iTunes. It originally was released without Mac support, but they recently made the drivers available. It has worked flawlessly so far. I have not used it in any other situation or with different types of files.
Generally, I am of the mind that quality digital sources are not so expensive or hard to come by and have struggled to identify significant differences among the DACs I have owned- although I have mostly used reasonable quality pieces of equipment. As such, I really do not have much to add concerning the sound of the DAC in comparison to others I have owned. If something something sounded bad or off, I would definitely hear it with the new Stax flagship. I do not think there is another pair of headphones on the market that is so revealing. Everything sounds good to me and I am very happy with the purchase. YMMV.