Copper may be the "gold standard" but PEX is on its way to becoming the industry standard. Cost of materials and speed of installation make PEX a very viable material for plumbing. PEX is indeed approved for drinking water. It is suitable for temperatures from freezing to 200 degrees Fahrenheit. It requires far fewer fittings in an installation as it is not rigid and can be bent up to 90 degrees around corners. Installation also requires less skill as connections are made simply mechanically with a metal ring and a crimping tool. As for long term stability, the product has been around for 60 years.