PEX stands for cross-linked polyethylene. Larry Weingarten likes it.

Its advantages are many. It is not generally affected by water quality. It's freeze-tolerant. It does not corrode nor leach anything into the water. It tends to quiet plumbing and eliminate water hammer. It's substantially easier to install.

As there is no torch for soldering, there's no risk of burning your house down. And considering labor and parts, it costs less to install than metal plumbing. PEX pipe is lower mass and does not steal as much heat from hot water as metal pipe, allowing you to get quicker hot water.

PEX is smoother than any metal pipe, meaning you can run smaller tubes because there is less friction loss.

The pictures above show some of these advantages. Larry merely had to drill holes through framing and snake the PEX like wiring through the holes. An advantage of having one PEX tube per faucet in a modest-sized home is that the volume of water in the plumbing system is far less. Ideally, you only need to waste one or two cups of water to get hot water at a faucet.