I don't think Cowon are different to any other Manufacturer. They have the occasional DOA device but it doesn't seem to be that prevalent (Sunova seems to have been really unlucky). I've got the D & D2 and haven't had problems with either. You get the occasional software glitch which Cowon usually corrects with Firmware updates. Cowons have many idiosyncrasies due to using older parts or connections but the sound is rarely condemned by users.
A lot of the time it is the person who actually buys it that's at fault, not knowing how to use them or expecting usability to be like Android. I bought my D2 as an open box for £160 and there's not much wrong with it bar a few software glitches which firmware updates should correct. It still had the film on the screen and looked like it hadn't been touched.