LED replacement bulb for C & D size battery flashlights was easy to change out. LED bulb is much brighter than the old incadescent blub which is what I wanted for my wife's car & our emergency kitchen Maglite flashlights.
Agree these bulbs are amazing. Gotta be careful though, because they need so little power to run that the batteries become discharged to the point of leaking. This is a problem when the battery cells get stuck in the Maglite.
I picked up one of these by chance, curious as to how it differed from the 1 Watt LED mod by Nite Ize and discovered that it has made a brilliant "survival" light mod. Light output is relatively low however adequate for seeing in the dark, and results in exceptional runtimes for 2D flashlights or higher. I used mine during the tornado and weeklong blackout here two weeks ago it made a reliable and long-life survival light. It's still using the same batteries and has been since last year.
Got the LED bulb upgrade for my 2D maglite before mag made LEDs. Changed the batteries after 2 years because I could not believe it was still running. On second set of batteries in 4 years. Still works like new, using every day for 2 minutes. They use less power than the real Maglite which I like.