Pros: Dynamic, aggressive sound
Cons: 24/96 optical input only, poor with IEMs, overheats when horizontal, newer model has poor amp
Update (1/19/2013): Don't use the HDP in horizontal mode, as it will overheat, and you'll have to send it in for repair. I learned this the hard way.
Having owned the $100 NuForce uDAC1 for over two years, I felt it was about time to upgrade. It had been acceptable, but I could still notice a bit of a veil in the midrange. The itch was also caused by going to a Head-Fi meet. When I plugged the uDAC1 into some high-end amplifiers, I realized how limiting it actually was. What does $500 (Canadian price) get you? A huge improvement.
The HDP supports 24/192 coaxial input, but only 24/96 optical input. Most PCs have an optical out jack on the motherboard. With it, there isn't really a need to use the USB input. The floor noise is dead silent, even with the volume knob turned all the way. I couldn't say the same about the uDAC, which had a faint hiss at maximum volume.
As for the sound, the HDP offers cleaner bass extension than the uDAC. However, it's really the mids that improve greatly. The uDAC almost had a u-shaped response, with a roll-off at the end. The HDP brings out the mids without any sort of a veil, which helps with vocals quite a bit. When compared to other dac/amp combos, this sound signature is certainly on the bright, aggressive side. Those who like this sound signature won't mind, but those looking for a darker-sounding signature should look elsewhere.
One important feature of the HDP is the analog input. I frequently use my computer's "Stereo Mix" option for when I'm streaming my computer on twitch.tv. The Stereo Mix feature on most computers does not work with S/PDIF, as Stereo Mix needs to pick up analog output. In order to enable Stereo Mix, I need to line-out from the motherboard's integrated audio jacks. This means I don't get to use the HDP's excellent DAC, but at least it works. With the uDAC, I couldn't do this at all and had to physically plug in my headphones into the integrated audio jacks. That meant the sound was very soft, as there was barely any amplification.
A large downside is that the HDP doesn't work well with certain IEMs. Plugging in my Sennheiser CX980 caused a nasty hissing, even at minimum volume. Despite NuForce's claims, this hissing isn't barely audible; it is very prominent. The uDAC was a slightly better match for my CX980, though it still hissed.
Having heard setups valued at several thousand dollars at local Head-Fi meets, I'm happy with my HDP purchase. This comes pretty darn close to them at a good price.