I have one three LED blinky attached to my rear rack and a one LED red "SpokeLit" light on my rear wheel (http://www.amazon.com/Nite-Ize-Spoke...le+wheel+light). I run them at night and when visibility is low.

I find that a bright headlight and a reflective vest make a much bigger difference in nighttime driver behavior than a rear blinky. The reflective sidewalls on my tires help a bit too, but it's tough to keep them clean.