I've bought countless (ok maybe like 20 or 30) E01s that I've given away as gifts. Every time I see someone I've given one to they say, "I had no idea how useful a flashlight could be!" because they have it on their keys all the time. My gifts have lit the way through blacked out DC subways stations and in otherwise un-navigable stairways. It takes a AAA and has regulation circuitry to make it last a really long time on a single cell at a useful brightness. Expect at least 10 hours at max and roughly the same brightness at half.

If you want to spend more money, buy American, and get an arguably tougher light, the ARC AAA is the way to go. It'll survive anything you can. It also only runs for about half the time, but it is awesome none the less, and what I have on my keys.

I’d recommend www.batteryjunction.com for the Fenix and www.archflashlight .com for the ARC AAA.