I had to make a trip to Lowe's today for some lightbulbs and a new step ladder, and saw that they had the LED upgrade for 3-cell Maglites. This is been something I've been looking for, as we have a 3-D cell Maglite at the house, so I added it to my cart.
The upgrade from the old krypton bulb to the new LED was easy. Just unscrew the head, unscrew the bulb retainer, remove the bulb, and reassemble with the LED. It took just a couple minutes.
Maglite doesn't make specific claims on their website or the packaging about how much longer the light should run with the LED, nor any specific claims about improved brightness. However, it should increase battery life, and it is quite a bit brighter (I checked brightness with the room lights off before swapping out the old bulb).
Like the krypton bulb, the illumination pattern with the LED can be changed by focusing the flashlight. The light color is different. While the krypton bulb provides a yellowish light, the LED emits a very white light.
Lowe's has the old fashioned 3 D-cell Maglites for about $18 - $20. The same light with an LED goes for about $39. The LED conversion bulb sells for about $19. I think it's a worthwhile upgrade for non-LED Maglites.