The sound quality is amazing. I have compared this with HD 600 and, even though HD 600 is clearly superior, the sound of the PX 100 ii holds up rather well. It is a warm sounding headphone and performs well with brightly mastered music.
The sound stage is small. So small it isn't even worth talking about. But the sound quality makes up for it easily.
All it would take for me to be happy is a version of this that's designed to last. I doubt Sennheiser cares, even though this is the most frequently (if not the only) raised concern with these wonderful sounding headphones.