My Brief 1st Impressions:
This is an audible upgrade over feeding the headphone-out of my Macbook Pro into the A2 and A5 speakers with stands. The treble extension and presence is a little better with UD100 vs with the MBP alone or with my original uDAC, with a slightly bigger and less forward soundstage too. In the past I've written that the original uDAC was just a little too dark with the Audioengine speakers, and I had switched to the DACport until I got my uDAC-2 and Audioengine D1. The uD100 is a good match for these speakers.
But overall, the uDAC-2 and Audioengine D1 line-out both seem to have a little bigger soundstage than the uD100, with the DACport surpassing even those in soundstage depth and size. Still, for $50 it shouldn't sound this close to the $130-170 DACs via line-out. These all seem to have fairly comparable levels of detail (not including the original uDAC). I do not think the UD100 is in the same league as my HDP with S11 PSU. I know the S11 PSU makes the HDP sound more refined and holographic, but I can't compare things with the stock PSU right now because it's been misplaced. Based on my past comparisons of stock HDP to the uDAC-2, and knowing where uDAC-2 stands vs the uD100, I don't think I would say that the uD100 is on par with the HDP as a DAC only. Still, the UD100 is surprisingly good for the price (and size).
I do really wish that the volume could be controlled by the computer, or that it had some kind of physical volume control. Instead I have to reach behind the A2 speakers to adjust the volume, or use a Westone in-line attenuator between the DAC and the speakers, which hurts clarity slightly. Compared to my old $49 Headstage DAC cable (a PCM2706 by itself) the sound is also miles ahead. But at least the Headstage DAC cable can have the volume adjusted via the PC system volume control, and it's using the same USB receiver chip as the UD100.
I bought the same TF card reader to transplant the DAC into later, but TF reader showed up in lime-green instead of black, so it's a little more fun but less professional looking. No pics until I get this DAC installed. I'm going to try feeding the UD100 into my SR-71b balanced amp in the next few days, and try it with my LCD-2 and HE-500. More to come later (when I feel like it).