I have a customer who is an engineer and has researched just about everything going into his new house. He has raised concerns about both the safety of PEX from a chemical standpoint and also potential problems with water pressure. See the links he sent me below. I'm a GC and lean on my plumber to make sure the water lines are the right size and all but I've never seen him do any calculations or anything. The last 4 houses I built(and several I worked on before that) all had PEX and I've never had any complaints from my customers. In this area at least it seems like it's all anybody uses unless copper is specified in a really expensive house or something. Is there another water line alternative other than copper? I guess there's the PVC type but couldn't it have similar chemical issues and is it cost effective?