Nope, this is just another "hi-end scam".

Doesn't sound like a scam to me.
Unlike funny rocks or little dots, there actually is some Engineering behind this.
Oh, wait.
This is the internet, everyone thinks they're an expert.

Cable Inventory
Kimber KCAG interconnects
Nordost White Lightning interconnects
some homemade stuff
some Monster Cable digital

No wonder it doesn't sound like a scam to you?

I don't mean to offend, and I think you're not taking me quite serious anyway But i'm just not one who believes in this kind of products.

best to reserve judgement till after you have actually heard it.

Agreed, I wouldn't put my money on this either unless some one brought it to a can jam and I could HEAR it with all of my own gear, in addition look how the review is written. It reads like an advertisment and seeing as the fellow who wrote it has a "review site" I smell a profit being made here maybe. Non the less the concept is valid. That power reason is exactly why I use USB 2.0 instead of 3.0, 3.0 is regulated for more power out so my dac get's TO much and sounds bad, where as 2.0 does not run as much power ect... ect...
Non the less props to you for writing it I suppose, in addition
like cusomt nuts said, "till you've actually heard it"

Another review here: http://www.audiostream.com/content/ifi-idac-iusbpower

Yes, the concept is valid.
SMPS (Switch Mode Power Supplies) often conduct and radiate a fair amount of noise.
Even linear power supplies create noise when the AC is converted into DC. It is a fact of life. You can't get rid of it, you an only reduce it down to an acceptable level. When the computer designers specify the USB power supply they are not thinking about you, they are thinking about $$$.
After working in an Engineering Dept. along side SMPS designers for 11 years I may actually have a bit of insight into this.
Does this thing work?
The final, definitive answer is: it depends on how much noise your source USB creates and how much noise your load USB device filters/rejects.

Question: if the price of this box is "only" 175 pounds, why is it not included in the design of any hi-end dacs?

Lol, I am just about speechless to hear all of these.
First of all, I am not making any profit out of all of the reviews I have ever written.
Secondly, the iUSB do give better sound, the significant changes is the 'nuance' and fullness of music.
Third, There is some issue with USB 3.0 to work with iUSB, it will caused some noise, in iFi iUSB,iDAC and Musical Fidelity Vdac II.
I just want to say, before you really hearing one of these, don;t ever came to conclusion that this is a scam or something, I heard the differences so I just stated that in the review. I am not denying anyone, just trying to clarify myself. And honestly, no single cent of profit that I have earned over this, it is truly a demotivate phrase that I have ever heard.
Anyway, thanks for the input, and sorry it took so long for me to reply here.

Headzone : Power supply for DAC/amp is always important from day one, that is why in some high end stuff there is another separate unit of power supply, clean power source prove the sound most of the times.
So this is why we can see some audio equipment manufactured power regulator and power regenerator to make sure the DAC and amp has clean and sufficient power supply, I have heard those and I can say it is actually improve the sound by much.
Now come back to iUSB from iFi Audio, it is more or less work in the same way, I have also said in my review, when I paired with device that has separate power supply unit, the difference is not big.
I am in Engineering field as well, however, for sound and audio, there is no TRUTH until you have really heard them.