I've had the UD-H01 for a few months now. The balanced outputs are connected to a Little Dot MKVI+ and the single ended outputs are going to a Schiit Lyr. It's connected via USB to an HP xw4600 workstation running Windows 7 Pro (32-bit). I'm also using Teac's HR Audio Player.
The only other DAC I've used has been the Teac DS-H01, so unfortunately I can't compare the sound to the more popular DACs mentioned in this thread.
I spent many, many hours researching balanced DACs and eventually narrowed it down to the UD-H01, Emotiva XDA-2 and the Musical Fidelity M1DAC. In the end I just couldn't bring myself to drop $800 on a DAC. At least not at this point in time. And I kind of like the idea of the dual DAC chips (that the Emotiva doesn't have) since this was primarily for a balanced amp. Also the Burr-Brown in the DS-H01 sounded really good with the Schiit Asgard I used to have.
Anyway, all that to say, it's worth looking into if you're in the market for a new DAC. I think the build quality is excellent, even though people didn't seem to like that the casing is plastic. It's a high quality / solid plastic.
Installing the drivers wasn't an issue either. Although it sounds like the original set of drivers were buggy. The ones from 12-28-2012 on their site are fine. Though I haven't tried them on an x64 machine.
And... It doesn't have an external brick or wall wart. I can't stand those things.
The only thing I don't like about it is the headphone amp. I just wish it didn't have one so they could knock the price down a little bit.