I have been looking at that little DAC as well since I listen to a lot of computer audio these days. I wonder how it compares to the Dragonfly. I feel Audioengine makes some great sounding products. I really like my A5's and A2's.
I haven'tried it myself but the D3 was selected by one of the reviewers on the site audio360.org as a "stocking stuffer" for 2013, which included a bit of a comparison with other similar dacs, stating that "Amazing bass definition and ruler flat frequency response sets this converter apart from the rest of the pack."
The complete text is as follows:
"The latest USB stick-sized DAC and headphone amplifier to hit the market is a welcome addition. From the guys at AudioEngine, known for their budget-oriented wireless digital and desktop loudspeakers, this little guy plugs directly into your computer's USB port and packs quite an acoustic punch.
Amazing bass definition and ruler flat frequency response sets this converter apart from the rest of the pack. The difference between the D3 and a computer's built in headphone jack is night and day."