They help. I wouldn't rely on them to replace lights, but they certainly help. It's best to put it on moving parts - crank arms, rims (the inner face, not the braking area! - because the motion identifies you as "bicycle" rather than "lamp post."

I usually use DOT reflector tape (the alternating red-and-white stuff you see on the sides of semi trucks) which I can get cheaply at auto parts stores. It comes in 2 or 3 foot lengths and I just cut it up and put it where I need it - white on the front and red on the back usually.