Just bought one second-hand... sounds very nice so far. This is my first true DAC/Amp unit, so take my impressions with a grain of salt.
I'm playing music off foobar 1.1.5 with the SoX resampler on at 96 kHz (the U2 can't natively play 88.2 kHz music), passing through ASIO4All v2 to the U2's USB receiver. I also have DA&T's custom ASIO driver installed, but it doesn't seem to work any better than ASIO4All and tends to lag my equalizer visualization.
Initial impressions are that the sound quality improves dramatically with high-resolution music. I lent my full-sized headphones to a friend, so I'm listening to it through my DBA-02. The LD output is nice and rounded, giving a gentler output compared to the HD output, which has a bit of a line level hiss coming through my sensitive IEMs. My quick listen to the HD output showed that it is more detailed and extended, however. As for now, I've been mostly listening with LD output because of its less fatiguing presentation and lack of hiss.
Soundstage is not especially large, especially compared to Firestone Audio's new ILoveTW asynchronous 24/192 DAC (the ILoveTW has a very large soundstage, almost too large to be true to source). Rather, it is about the same size as the Audinst HUD-MX1, but with better texture and transparency, as well as a sense of power not found in the small Audinst.
In terms of styling, it is most certainly one of the most handsome of the recent barrage of budget-priced DAC/Pre-Amp/Amp combo units out in the wild. The casing is a very solid, black brushed aluminum, and the faceplate is thick, clear acrylic. The standby/on button is touch sensitive, instead of a physical button, which gives the entire unit a touch of class, but is prone to accidental pressing. The volume control knob is a tall, tapered frustum design, held together my a hex screw (which presumably allows you to change it out if you so please). The tactile response is pretty good, gliding smoothly but with ample resistance as well.
Overall, I'd say that in terms of looks and ergonomics, only the Nuforce Icon HDP and the Burson Audio H160D outstrip it. It is a far better looking model compared to the Firestone Libby, and by all accounts (I have not heard the Libby myself) the U2 is the more sonically neutral unit of the two. I have no experience with the Yulong D100, but the U2 is unequivocally a better looking model.
All in all, for $300 (list price is around $400 USD), I think I've got a pretty good product in my hands, from the craftsmanship to the sound.