They seem to be of good quality and are comfortable, and the sound was good when connected to the sound-card of my PC, however the volume is woefully inadequate when used in conjunction with my Yamaha P-525 digital piano. Key to this - I discovered - is not just impedance but more importantly the sensitivity... even a small drop in the sensitivity rating means they need significantly more power to drive them.
I tested 4 headphones: Yamaha HPH-150B, AKG K-702, Beyerdynamic DT-990, and Sennheiser HD 560S. Only the Sennheiser HD 560S produced plenty of volume (plus it had higher impedance, generally giving better detail).