I've owned a Cowon D2, and currently own a 5.5G iPod and a Sansa Fuze.
The D2 had good articulation on mid-high notes, but low notes sounded uncontrolled, it had small soundstage by default also but this was correctable with a rather effective effect (can't remember the name, but suffice to say it sounded fine with it enabled). It also had a EQ which didn't extend low enough to reach the subbass, 80Hz was the lowest band meaning if you EQed it up at that point, it would sound mid-bassish instead. It also sounded a bit harsh because of the articulation, back when I used it with my iM716s, it was particularly bad and I ended up selling it for mostly that reason and the lack of subbass.
The Fuze has an average soundstage and a pretty muddy sound quality. Going from either the D2, an old iRiver H320 or the 5.5G iPod it muffled detail considerably more. The EQ was also unusable because it would cripple the rest of the frequency signature and again didn't extend into subbass either. Sound signature was fairly balanced though. I would have preferred more subbass, less midbass, and slightly more detail resolution in the highs though. Definitely didn't sound harsh though like the D2, has a warm sound signature. Anyway, overall it was average, I don't get the ravings about it here, I think it's like and sounds like any other budget player.
With the 5.5G iPod, the disclaimer is, I can't even remember what it sounds like flat with no EQ and Apple firmware. I have a pretty heavy sub-bass EQ for my ER4Ps and it sounds great to me overall. Detail resolution is excellent, slightly better than the H320 I would say which is more towards the warm side of the sound signature, but not as much as the Fuze. It can be relatively harsh like the D2, less though. May have something to do with the fact that the ER4Ps are already fairly harsh sounding I think this can also be fixed with a carefully placed EQ in the high frequencies but I haven't figured it out. Soundstage is also very good to my ears, better than any other DAP I've tried. But yeah, to me the biggest selling point is the rockbox EQ. You can get it to sound like anything you want. For example I recently noticed that EQing down at 600Hz very slightly allows me to extract more detail. Nifty things like that really sell it for me.
Crap I wrote an essay!
