Many have asked about these but the jury is still out. Some people have posted that the pulleys are already really light so there might not be too much gained. Some have also posted that if the crank pull gets any lighter there will be a balance problem. I don't want to be the car for them though.

03.5 Sedan/B. Silver/Prem/Winter/Aero/Tint/Rims
69 Chevelle Hard Top BB/4.11's/Posi

 

This is true harmonic balancer can get affected if it gets to light.