I love mine. Only a few gripes so far:
1) once you add the split ring, it doesn't attach to the headlamp kit any more. Anybody found a way around that?
2) Takes 16340, but a) my protected cells have a tight fit and lose the high mode b) my unprotected cells go off>low>off>high. I don't know if it's some kind of issue that may eat up battery or be dangerous from shorting (or may even be useful this way), so clarification would be good.
3) Only had it about a day and the oring gets snagged by the head some times.
Aside from this, it's amazing. I use it on my necklace. Somewhat heavier than my old Nano-copy, but the gains are amazing.