One thing about these lights that you should be aware of...

I used to carry an Arc-AAA light. Those use type III hard anodizing, and I kept it on my keychain. After a few years, the hard anodize combined with the knurling wore down some of the bumps on my car keys, and I had to replace them. I now carry a Photon light. Yes, it uses coin cells, but the power output is adjustable, and I have a pile of coin cells that I can replace at any time. It is not as nice, but it is a LOT more gentle on my keys. My Arc-AAA now lives in my camelbak.