Well the post above claims that the iFi contaminates(?) your mains for some reason, if really true that may be a problem depending on your set-up.
With a D50+dedicated amp set-up I personally wouldn't want to use the iFi though, plain USB power works fine according to amir's measurements, and while battery-based or linear power supply-based 5V supply could offer mild improvements to the noise, the range this noise occurs in shouldn't really make for audible differences. Which makes me question if it's really worth investing in some exotic power equipment, the Topping D50 is what, €200? Why invest in €50+ of power supply when your average laptop USB port is really, fairly clean all things considered?