I've got a Nano Light on my keychain and it has seen plenty of use over the last year or so. The output from this tiny thing is impressive. I have used it as my primary flashlight during some evening strolls, but more often it's what I use when poking around my car or house at night.
It's not perfect. The black coating did not last very long, being attached to the keys and all. I have not had the batteries last anywhere close to the advertised 8 hours, but that could also be a function of using cheap batteries combined with riding around in a pocket most of the time.
In my experience, the light would start to flicker over time, regardless of how much it is actually used. A fresh set of batteries fixes the problem for a couple of months. I also wipe the new batteries with a dry cloth just before putting them in, and this seemed to lengthen their useful life.
All in all, a very compact powerhouse for just a few bucks.