Just a short review, which likely repeats much said by others. My time with the D100 mkII was cramped, in that the timing was particularly bad. Work kept me busy throughout the week and my two weekends were filled with a trip, two birthdays and the California Audio Show (where on the upside, I met Wes). I was able to carve out a couple hours two evenings, which made up the bulk of the below impressions.
Although when I signed up for the loaner program I had five DACs, in the end I was down to one, the Lavry DA10, thus my comparisons are rather limited. Most of my listening was done with a Squeezebox Touch, Apex Peak + Volcano (w/Shuguang Treasure CV181-Z) and Sennheiser HD800s.
Starting with Alison Krauss & Union Station's Lie Awake, and then continuing through the rest of Paper Airplane (16/44.1 with Lavry volume matched at 47-48 depending on track), lead vocals on the D100 were less forward, while the backup vocals (when they existed) were less recessed/rose in prominace compard to the DA10. I wrote at the time of the Neko "Less peaky. Less dynamic."
Other times the differences were far less pronounced, such on Ebb Tide from a 16/44.1 needledrop of Illinois Jacket's Birthday Party, where the DACs performed nearly identical.
Overall through my experiences leaned more towards the former results and backs up others comments, most commonly that the Neko D100 mkII was very "analog sounding" This was confirmed more recently when my first turntable in two decades arrived, a Rega RP1 (w/performance pack). Again my base, the Lavry, at similar volume or without attenuation, was more aggressive. Several times I thought - if one wanted most of vinyl without the surface noise (especially for us headphone users), the D100 might just be a great choice.
The D100 also reminded me of the ill-faded (plagued with customer service issues) Aeolus Audio magiDAC (not to be confused with the more famous Cambridge Audio DacMagic). If memory serves, the Neko excelled beyond the magiDAC in every way, but there was a similar gentle top end that would surely appeal to many listeners having issues with digital audio.
I really enjoyed my time with the D100 mkII and wished I had more time with it. If you're looking for a very detailed, but softer sound, this could be your path. I do want to warn though if you've already built a system around a DAC with higher output (most of my recent DACs have had) or more dynamic sound, you may want to consider the synergy question. Although sounding great with two headphone setups, I did notice extension issues with my speaker rig (which has extension issues of its own). This of course is a danger with any gear switch and should not be thought of as a deficiency on Neko Audios part. Much like the larger "analog sound" target, just be aware of the associated ramifications.
Finally, if I can make one small feature request to Wes it would be to add a sampling rate indicator. The D100 is far from unique in not having, but once you're accustom to using it for everything from system debugging to album version checking, it's missed when not available.
Thanks for the listen.