I formerly carried a photon on my keyring, but some years ago I switched to a AAA light; I have found the latter to be far more useful. For one thing, a twisty AAA is far less likely to be accidentally turned on, compared to a photon. I equip the AAA with a lithium battery, thus having a bright stable light for the duration of the battery. A backup battery is always in my EDC. One battery typically lasts about a year in typical service. I have carried both the Peak Matterhorn and, currently, a Fenix LD01 - I prefer the variable light levels of the Fenix.