Its not the best but it isnt the worst either. I just use mine at my desk, had them for 5 or so months, seems to be fine still.
I love the 598s, I own HD 518, HD 598, and HD 650, and the majority of the time I'm using the 598, they are very comfortable for long periods of time, more than the 650. Also, they can play straight out of a device, like and iphone or whatever. But, I recommend a Sennheiser or Grado 1/4 to 1/8 flexible adapter to go with them.
HD558's for sure. They are exactly the same as the 598 after the foam mod as both headphones have the same drivers. About 200hours of burn in and I like them better than the HD600's