I like them but don't find them that helpful. If you are walking around in the dark, turn your black flashlight off, you still won't be able to find it with a GITD tailcap because it will have had no light on it for a while.
A GITD o-ring around the front lens is possibly helpful since the light itself was "charging" it.
I just like the pistachio colored GITD tailcaps because they look good on black flashlights and could be useful in locating the light if the power to your house were to suddenly go out and if your light was sitting out (and not tailstanding). :)