HD650 is the headphone people love to hate (as evidenced by this thread); even when they have no direct personal experience of it, that doesn't prevent some from having strong opinions of it. Based on nothing but somebody else's opinion (which may or may not have much validity) in some cases, but whatever.
Its success speaks for itself... How many tech products (especially in the fickle, 'wonderful today, trash tomorrow' audio world) can claim a 20 yr track record?
BTW 1997 was the 580. 600 came later and 650 a number of years after that. The 'gist' of the design may have originated with the 580 but there many design detail and sonic differences between them. The quality of the original design is why it has been so successful for so long. 600 and 650 do not use the same driver. (Tyll H did a comparison at innerfi; the plots look quite different). BTW this line just entered its fourth iteration, with the 660S. Is it better? I don't know; I lack the psychic ability to judge audio components I haven't heard. I suspect it's probably not too shabby. It's definitely competitive in it's price bracket, if past history is any indication; if you don't like it, there are plenty of alternatives. If there's one thing that galls me more than presenting opinion as fact, it's repeating somebody else's opinion and presenting it as fact.