I'll throw my $0.02 in here...
New to Head-Fi, but I've been anonymously lurking (and listening vicariously) through y'all for a while now. Music has always been a passion for me, I listen to music ranging from rock to heavy metal, with forays into other genres (excluding most pop/rap/rnb), and I have been playing guitar for about 14 years (electric primarily, and not with your $300 xmas gift box set guitar/amp combo), so I like to think I know how I like my music to sound to some extent.
Now, before I say anything, I'll say up front that I definitely wouldn't classify myself as a hardcore audiophile, the only semi-serious headphone I own is a pair of TF-10's with the RC-UE2 FiiO replacement cable (more for comfort than SQ upgrade), others include a JVC FRD-80 (which I bought after reading the reviews on here, for the bang-for-buck), Sony MDR-1RBT/MDR-10R (I'm a member of Sony X in Australia, so I get discounts, which makes Sony gear much more appealing, slowly converting me into a Sony fanboy), very recently a Sony ZX1, and miscellaneous cheap IEM's.
Given that I came from listening to music through my Xperia Z Ultra, the ZX1 was a notable upgrade, crisper and with more definition... But in my humble opinion, purely based on SQ, not $560 AUD worth of upgrade. The experience on the whole, definitely worth it, the solid chunk of aluminium is glorious; the ZX1 is one of those devices that gives me joy from just holding it. Plus I got my MDR-10R bundled for free with it as a special pre-order deal, so that definitely helped seal the deal.
So on the whole, I would tend to agree with eke2k6, though I would put it more in terms of the law of diminishing marginal returns (I'm an accountant by profession/training, so excuse the economic rhetoric): most baseline players (iP4, iPod, etc) I imagine would sound pretty good these days, more than enough for most of the masses. The jump from them to the cheaper end of the "audiophile" grade devices, for most people, would be noticable but slight improvements, and the jump from those to the even higher grade devices would be even slighter improvements. Whether the improvements between them is worth their price tags is purely dependent on the person on the listening end, and their perception of value.
To put the prior paragraph a different way: would you be willing to pay $600 to have crisper trebles and fuller bass lines in your music? $1200 for a wider soundstage? It's a rhetorical question, as everyone's perception of the differences in the SQ and how they quantify those differences is unique.
Sorry for the long post, guess it turned into $0.05 from a noob