My Dh has G6PD. It basically mean that substances most people metabolize with no problem are highly toxic to him.
he was born before these things were screened for. It wasn't caught till MIL had to rush new born Dh to the ER. he almost died.
Ok, here's my two cents as a mom with a pku baby(he'll be two next month). We are really glad we had the test done despite initially being apprehensive about it. The disorder is incurable, the effects irreversible. It was really, really hard at the beginning and my heart was bruised and battered but things are leveling out and he's mostly a normal toddler. We've got a few neurological effects that are being looked after but he's ahead developmentally. If we had opted out, we did have the option, he would be seriously debilitated by now. I had the opportunity to meet a few moms who opted out and now have very seriously retarded and physically challenged toddlers who will wear diapers for the rest of their lives. It's a really hard road having a kid with pku (at least this is our experience, in this state, but that is for a different thread altogether) but knowing before the damage is done is so much better for everyone in the family.
I also have a child with PKU and I'm so very grateful for the newborn screening. Had he not been screened he would have suffered severe mental retardation by the age of 1.