Pros: good frequency response
Cons: muddy bass, soundstage not very wide, slight veil
I recently used a borrowed HD650 in a balanced system for about a month.
After I gave it back and returned to my balanced Grado HF2's, I was shocked how much better the Grado's were.
I must thank Sennheiser for making me appreciate my current headphone. That's about the only good thing I have to say about the HD650. :) I'm kidding, it's good but if you value an open and fast sound this is not the headphone to get.