i've bought a bunch of them; they make good gifts. i don't have an easy way to measure the output, but in my office with the fluorescent light on, the beam center (maybe a couple of feet across) will easily be seen on the wall across the room (maybe 12-14' feet away. with the lights in the room off, the spread of the beam easily lights up the floor close to me with the body of the light held horizontal.
the light itself casts a bluish colored beam - but in the dark, that's probably easier to see.
one gotcha - in the ones that i bought early this year, the end would unscrew while on my keychain. this results in the 4 LR41 cells falling out. i just took some teflon plumbers' tape and wrapped it around the threads - now it's perfect.
it's listed as having a 10 lumen output, with 8 hours of run time. that sounds about right. it's not a whole lot, but in a light that's no longer than the middle joint of my index finger (and a LOT smaller in diameter) it's pretty good. i always have 4 or 5 lights with me, and having a nano as a back- back- back- back- backup is reassuring.
oh, and the idea of it as a zipper pull is genius!! just be sure to teflon tape the threads.