I bought and installed a set of Percy's with my Hooker headers about 4 years ago. Haven't had a single problem with them. They were well worth the price.
There is one thing to remember though, every so often you should check the bolt torque. Every time I've checked my bolts I've found them still tight. There is no gasket recession so the bolts have stayed in place. Vibration, obviously, could cause the bolts to back off, but it hasn't happened to me.
I've had the headers off once and the Percy's are still in good shape. You always want to keep the gaskets in their original position once they have taken a "set". They'd probably still seal OK if you mixed the positions up, but why chance it. ---Chuck | 
Head Gasket Advice. Is Percys Seal-4-Good Reusable Flange Gaskets any good? 
