Gold Lifetime Member
Join Date: Oct 2001
Location: Londonderry, NH
Hardened valve seats are necessary for continuous operation on unleaded fuel. The lead in gasoline gets deposited on the valve seats and acts as a cushion to prevent the valves and seats from getting hammered especially with high pressure springs!
You don't really need that much lead to keep the seats happy, and now they make additives like Bardahl "Instead-o-lead". These are sulfur based products that perform the same function as the lead would.
If they are properly installed, I can't think of any negatives to installing the hardened seats. You will occasionally see a car "drop out a seat" and make a mess of the engine, but I think that this is rare and generally the result of bad machine work. (Just my $.02)