The very first time I handled a One I was hooked, the size, shape (curved back) and material was just sublime, and that was just a dummy display unit. Having said that and having owned the phone since it's UK release, I am sad to say that it now lives it's life 24/7 it a case, albeit an offensively expensive but supremely beautiful Noreve model. In an ideal world it would bask naked in all it's glory but until my employer replaces the concrete floor with squishy rubber, thats where it is going to stay.
I am no fan of TouchWiz but like Sense, it is easily replaced, so I could never use a specific interface as justification for not buying a device, but even if the (hypothetical plastic) One and the S4 both came with stock vanilla android, I would still choose the One. Most software dependant features of the One could be replicated on the S4, Blinkfeed with Flipboard, custom eq apps for beats, even to some degree some of the camera features, but there is no way for the S4 to replicate the effect of tthe forward facing speakers or the universal nature of the ir device, which are two things I use on a daily basis that give me far more functionality than all the eye tracking technology in the world could ever do.
I handled my first S4 last night and while the amazingly tiny bezel around the screen looks very impressive, it actually makes it horrible to hold, there is no way to grip the phone securely without having your fingers on the screen and that, in a nutshell, sums up what Samsung is all about these days, function over form, technology for technologys sake. We are going to make the bezel around this screen as tiny as we can, not because it makes the device better, but just because we can.
If it was made out of cardboard, the One would still be the better overall package.