I bought the Nuforce Icon HDP six months ago. I needed an amplifier to drive the AKG K702 headphones that I was buying at the same time and I decided to buy this product since it also contains a high end digital to analog converter that can be used with other amplifiers or powered studio monitors. The price was around 450USD and is the same at the time when I am writing this review.
This is such a flexible device because the converter and the headphone amplifier can be used independently of each other; which can be hard to see from the specs. The input mode for the device can be selected using the second knob at the front (the other is volume with volume=0 turning off the device). These are:
-U: for USB mode, where the device acts as if one connects the usb port in the back to a computer. (cable included)
-D: for Digital input, from the optical line in (TOSLink, which macbook pros have) or coaxial at the back. cables not included.
-A: for RCA or 3.5mm input. bypasses the digital to analog converter. cables again not included.
For output, the device has 1/4" headphone output at the front and RCA analog outputs at the back (which bypass the headphone amplifier of course).
As for sound, I am no expert on audio quality. For headphones, I was using Sony MDR-7506 headphones prior to the AKGs. I tested both these headphones and my monitors with the DAC and having it makes a noticeable difference compared to the macbook pro's converter. A much bigger difference than my apogee Duet's converter for the monitors.
For the amplifier, the AKGs which have very low volume on regular computer outputs, can be very very loud with the amplifier, alltogether I can hear things more clearly and quickly when mixing, when I have to use headphones (which is all the time now as I had to get rid of my monitors when moving to Europe). But even with the Sony's the bass and the mid frequencies are much more defined when using them with both the DAC and the amplifier. I must say overall it makes music more more pleasing even on the Sony's.
There is one little annoying thing, which is that even if the device is off. It still draws power and gets slightly warm. Nuforce have since released a fancy linear power supply, which also has an off button, for use with this device. But I don't think I will be buying that, so I bought a plug with an on-off switch so that I don't have to keep unplugging the power supply.
To conclude, this product (together with a headphone upgrade) made my listening and my music making more enjoyable. I would recommend this for someone who wants a good headphone amplifier and a digital to analog converter. I think it's always good that the converter can be used by itself.