I can tell you, that as an iPod owner and not much of an audiophile, that you would probably be disappointed in the sq of the iPod, coming from a Cowon player.
The iPod is "just" good enough, unless you decide to go the route of getting a lineout dock/cable and using a portable headphone amplifier. As I value convenience pretty highly, I have chosen not to do that.
I guess you could get used to the iPod's sound after a while, but if sq is very important to you I would look elsewhere. I think I heard mention that Cowon is coming out with an X7 this year.
The other players I'm personally familiar with are the Zune, and the Sansa e200 series (I also had a Rio Carbon, but that is a long discontinued model.) The Zune's sq is fantastic. Unfortunately, the software you're required to use is terrible (no MC support) and the player (imo) is big, heavy and ugly. The battery life is poor, too (8-10 hours).
The Sansa series is flash-only and limited to 8gb. It's sq is worse than the iPods (but not by much).
Rick