Pros: Small, solid, visually attractive. Sounds both as clear and as hard and strong as diamond.
This is exactly like the $100 more expensive Icon HDP. I'll quote the description on Nuforce's website: "The Icon HD is designed for users who want the Icon HDP's world-class USB DAC and headphone-amp performance without having a need for the Icon HDP's flexibilities"
It is an HDP stripped down to a usb input, a DAC, the RCA output, the volume control, and the 6.3mm headphone output. For a person desiring only a DAC/amp for their headphones and nothing more, this is a much better deal than the HDP which might have unnecessary (depending on user) inputs and outputs and features.
This is my first "real" amplifier. So, I cannot say a whole lot (please refer to the Nuforce Icon HDP reviews, as whatever is true for that is true for the HD as well, as the dac and amplifier sections on each are identical), but I can compare it to what I am used to.
Using it with Denon AH-D5000 and Sennheiser HD595 headphones, it sounds fantastic. I cannot determine any flaws. As others have said, there is no noise floor and this puts out great power for a full sound (no missing bass, no muddied highs, etc). It's exactly what I would expect a hi-fi amplifier to be.
Comparing it to my other amplification solutions, this thing is a giant above any of it. It beats the output on the HT Omega Claro Plus+ sound card in power, bass, and shockingly to me, even clarity - something I thought my sound card particularly excelled in. This is crystal clear compared to the Claro, which, while clear itself, just isn't quite as transparent as the Nuforce. Speaking of clarity and transparent qualities, the Icon HD completely wrecks every headphone output I've ever heard on any SOny/Pioneer/JVC/Denon stereo receiver I've owned or used. I haven't heard any super hi-fi receivers; only the more affordable ones, so the Icon's superiority is definitely expected and understandable.
I think a lot of people are content with their "cheap" amplification. They just plug into their sound card, or they just jack into their home theater receiver, and they call it a day. That is what I did for the past 2 years. Today, I realize that I have been missing something that makes a huge difference. I strongly encourage anyone without discrete headphone amplification to go out and get this - at only $350, and with all of the quality of the HDP, this is a steal.