I would like to point out from the beginning that I have not every tried any other portable amp. Also, I am using them with a very entry level pair of the headphones (monoprice over the ears DJ). A coworker bought the Fiio E02i, so I naturally was curious as to what it would do. While I was expecting more volume perhaps at the SQ, what I found was a substantial improvement overall. It makes my headphones sound twice as good. I naturally purchased my own. I have no doubt that the limitations of this amp would likely be obvious with quality source/cans. But, for very small $$, I am thrilled with the results. I realize that giving a rating is rather useless as I have compared it to nothing else.

***4/25/2014 Update***

I will now follow up after a bit over a year of use. I have upgraded to the Digitech Pro Monitor Headphones which are an enormous upgrade in SQ and especially comfort over the Monoprice DJ cans. The Fiio Rocky was easily up to the task better quality Digitech Pro Monitor Headphones. Just recently, the right channel stopped working on the Fiio Rocky. If I twist the cable near the 3.5 mm jack just right, the channel comes on. I have not abused the cable/connector at all. At $18, I guess I cannot complain too much. I am guessing that the very small gauge cable added to the eventual failure. Looking back, I am glad I bought it, although it really should have lasted much longer. I have adjusted my ratings a bit. I I do not think I will buy another Rocky. I am looking seriously at the E17 Aspen now.