This ticks so many boxes, with high quality amplification able to drive difficult loads,(it smooths out the low end in my Audio-Technica ATH-M30s enough to make them listenable), great size and weight, excellent volume control, and hardware switches for bass boost (it doesn't need it, but it is there) and output gain.
But there are a few design features that make it a deal breaker for me: I have one, like to listen through it, but rarely do because:
 

1. The unit cannot run whilst charging through the 5V USB port. So if you aren't charged and ready then you 'aint using this baby. I know it is likely to improve battery life, but major problem for me.
2. The unit drains considerable power when switched on, even if there is no signal voltage at the input. If this is turned on and doing nothing, the clock for 10 hours keeps running. Don't use this amp to go to sleep listening to something, or you will be out of juice in the morning (something I routinely do)

My only other minor gripe is the mismatch between the size of the unit and almost every mobile audio device excepting the older long ipod nano means the supplied rubber band method of clamping to the audio play is not useful. This is begging for a velcro fix method on most devices.
 
So with a few tweaks, I'd likely think this was an exceptional buy. It probably still is, for the amplification quality and the money, but for what I use it most often for (amplifying my go to sleep music) it suits very poorly.
P
 
Edit: (either not yet allowed to comment, or cannot figure out how to reply to comments) Reconsidered rating, added another half: I bought this in Australia when it was hard to find any retailer selling a Fiio, hence the high price, but as I said, despite being a great amp amplification wise, it sits idle on my shelf 99% of the time because of these flaws. I may as well not own it at all.