With the antiquated points ignition and, carburetor fuel delivery systems (both of which begin their gradual degradation until their ultimate failure due to normal wear and tear upon the first revolution of the crankshaft), these plugs will foul just as quickly as the original equipment plugs did.
With the advent of computer controlled electronic ignition and fuel injection, modern engines are not subjected to the problems associated with the old system, and therefore you can run plugs for pretty much the life of the engine.
Like Carl, states, I can't see any real advantage to them. If they are being used with a coil on plug application or even an HEI setup, maybe some modest gains. Nothing to warrant the price tag on them.
I'm willing to bet just going a couple points hotter, using an "S" type plug, or even indexing your spark plugs would probably have about as much of an effect as a $15 magic plug.
Buy some R45S plugs for $2.00 each and be done with it.
