Lead is poisonous, especially to young children. Its a heavy metal so once your body absorbs it you cant get rid of it.
Overexposure to lead interupts signals sent from your brain, this per my lead base paint class.

Last night on the History Channel- Modern Marvels they did a show on Gasoline. It explained octane, lead, etc.
They explained why lead was used in the beginning, and why they quit.

 

I agree with pretty much everything said here, but just wanted to add one thing since it wasn't already said. Don't run leaded fuel in anything with a catalytic converter! The lead WILL destroy the converter, and those can be pricey to replace.

Marc