I've had my 600s for about a month. I am honestly surprised at how light they feel. They don't feel flimsy by any means but they are surprisingly light. I'm really enjoying their sound and they are only just barely not as comfortable as my Sennheiser HD555s. I really love the oval shaped cans that Sennheiser uses.
I'm running them off an Audioengine D1, and I think I want something bigger because they seem quiet. Not sure what to get for them, though. I get paid Friday, maybe I'll pick up an O2.