It's mediocre in sound quality if you like coloration.
Rockbox doesn't even do anything to the sound unless you mess with the EQ and other settings.
"Digitized and robotic". I don't even have to comment
The headphone out is fairly powerful, capable of nearly half a volt into 15 ohms. It won't drive orthodynamics to ear-splitting levels, but it's enough for any headphone that prioritizes portability. It puts out more power than an iPod Touch before clipping. Its low output impedance means low impedance headphones of any kind are fed a healthy amount of power and remain uncolored.
The fact that you assumed it was weak because of its size is ridiculous. The Fiio E6, a tiny little portable amp, can handle 2Vrms into high impedance loads. That'll drive an HD800 to 108dB.
Edited by Head Injury - 9/20/11 at 11:21am