Well, compared to carrying around an extra battery and a USB cable, it might be better.
hmmm not really
if you use an iphone, you cant really hack the inductor inside of the case, so you will need to wrap the inductor outside the case ... AND the horror is, it will then need to be wired in somehow (or so i would think)
if you use an android, most of these can expose the battery, and there is a chance the inductor (or circuit) can be hacked to the battery terminals (or so i think)
in either way, if the inductor is made to supply direct 5v, then we are all out of luck and it will need to be plugged to the usual charge port .... (so there is a cable again
)
... if outdoors ... you will still need to carry the battery pack. and with the slightly reduced efficiency, the pack is giving less usable charge than a direct 5v plug in power pack.
but for android users, and especially samsung ... just buy an extra 3.7v battery. its more portable than anything of the above