This and That:
Comes with a nice case as well as a long micro USB to USB cable.
Looks nice; really good build quality. Small.
Didn't require any additional drivers to be installed (Windows 7). Seems like the volume buttons work as multimedia buttons and control the PC's volume with the same step size as the system volume (16 on my Windows 7 computer).
Sound:
Very powerful. Too loud for me using sensitive IEMs (Xiaomi Piston Colorful Starter Edition as an example, some inexpensive consumer in-ears) even when setting the system volume to 1%. I need to reduce the volume even further (foobar2000).
Frequency Response, no Load:
Unfortunately one can see a roll-off towards the sub-bass. Too bad; that shouldn't happen on any modern device. Not load-dependent.
Output Impedance (loaded, Ultimate Ears Triple.Fi 10):
The calculated output impedance should be around 0.2, 0.3 Ohms, which is an excellent value and suitable for all critical multi-BA in-ears.
Hiss performance:
Strong hiss even with in-ears that aren't near-extremely sensitive.
Subjective Sound Perception:
I'm clearly more an objectivist than subjectivist when it comes to audio.
To me, the D1 sounds just like any audio device should sound and doesn't show any flaws (except for the aforementioned bass roll-off and hiss performance using in-ears).
Conclusion:
Inexpensive, nice storage case, great design, high build quality. Low output impedance.
Too powerful for in-ears. Bass roll-off regardless of load; poor hiss performance.