I have the 500s. I paid something like $50 for them on ubid like nine months ago or so. Yes, they are nice. Audiophiles tend to think they have a "phased" sound to them... I dunno. There is a modification posted on headwize, which I did a few months ago, and I think they sound better, w/ increased bass output (it rattles your face!!).
I would like to get the 580s, because I've heard them and think they sound a little better than the 500s, plus they have velour earcups (drool...) instead of the vinyl ones on the 500s.
Don't get me wrong though... the 500s are very nice cans. They are comfortable, loud, and have a long, single cord, which is nice. The 580s, on the other hand, are connected on both sides, which is annoying, IMHO.
Hope that helps.
l2c