As noted before, the hf2s have a very refined, almost monitor-like, quality. There's none of the bass-heaviness found with some rival earphones, but that's no bad thing. Instead, what's on offer is a beautifully detailed sound which will suit the kind of listener that favours precise, uncoloured reproduction.
Vocally driven tracks such as Phantom Planet's California or The Saturday's Fall especially benefit from this. Backing music remains where it should be, in the background, not dominating the tracks as it can through some earphones.
Switching to Christina Milian's Dip it Low show that the hf2s can handle a bit of pop with aplomb. Although here I think the lessened bass is more of an issue. The haunting electronic beats of Crystal Castles definitely benefit from bassier earphones and Nine Inch Nails' Head Down lacked presence through the hf2s. A pair of SE420s, conversely, suffered from no such issues.
This disparity was fine in the standard hf2s, which were significantly cheaper than any multi-driver earphones. However, a pair of Shure SE420s can be had for less money as these hf2s and I do think they sound better, offering similar detailing to the hf2s but without the arguably 'sterile' bass. I don't have a pair of SE530s to hand to test, but at around £200 - only £20 more than these custom hf2s - I can't see them sounding anything but notably better.
That's the price you pay for the comfort and isolation offered by custom tips. Having splashed out on a pair of SE420s myself a little over a year ago, I'm now finding myself daily in debate as to whether to use those or the custom-tipped hf2s, simply because in the environments I'm using them, I'm generally able to get more from the hf2s, despite the technical advantage of the SE420s.
The hf2s also have the benefits of the in-line remote and microphone to consider. Being able to listen to music while out and about without missing phone calls is a luxury that is, if not hard, annoying to give up. Being able to skip between tracks on my iPhone without having to pull the thing from my pocket is similarly great.
Etymotic Research's partnership with ACS has undoubtedly created the best sounding pair of iPhone-centric earphones available to buy. Just, be prepared to pay a sizeable premium for the privilege of enjoying them.