Phew. Gotta chime in here on a couple of points.
The Cowon A2 is a little bright, but nicely detailed. It pairs nicely with the Future Sonics EM3, since they are low-impedance with massive bass, which serves to even things out a little. It's pretty sterile and bright with the Tomahawk, though, at least in comparison to the H120 line out.
The iriver H120 headphone out is *NOT* very good. Soundstage separation is weak, and hiss is audible with low-impedance cans (ie: IEMs). That said, the line-out to an amp sounds wonderful. Perhaps not as detailed as the Cowon A2, but with the Tomahawk it's much warmer and more musical. The A2 would be fatiguing, the H120->Tomahawk is not. I literally listen for hours at a time every day during work hours.
Rockbox is a godsend. The extra functionality it provides is marvelous. It REALLY gets to shine with the extra functions unique to the H120 (optical output, recording inputs, FM receiver, etc.). Gapless is a wonderful feature alone. I hope to use ReplayGain soon, too, once I've finished re-ripping my audio collection to LAME 3.97.
And to the guy thinking of switching to iPods from some WMV player: just encode with LAME VBR MP3s. WMV isn't as good anyway, and while AAC/M4A is a better compression than MP3, LAME-encoded MP3s are the most portable format available (ie: everything plays them). There's plenty of guides out there for ripping with EAC and LAME. Highly recommended.
I'm so curious to hear what one of those 2G Nano's sounds like. They're slick.
-Packgrog