I tried screendim before I found screen filter, but it didn't dim my phone any further than the normal min brightness. I have been meaning to try it on my transformer though. Actually I might use both even if it does work. My transformer is very, very bright. Nice in daylight, not so much at night.
I'm not sure how much life my phone has in it, and that's my primary reader. If my transformer does have to stand in until I get a note, I'll need to do something to make night reading more comfy.