Hell, I still have a DJM-500 and it's just fine. I'm trying to sell it locally because I have a DN-X1600 now, but only because I wanted a Scratch-certified mixer.
Honestly, I never had any complaints with sound quality. People complaining about sound quality when "pushing it into the red" are declaring their own problem. :)
If you can score one cheap, don't sweat it. They are tanks.