For the curious, here is what PEX looks like:
It is a sort of plastic tubing suitable for carrying fresh water. It is increasingly used in new construction. It is used in place of copper piping:
I’ve installed both, and they have their pluses and minuses.

Because I don’t do plumbing all the time, I long ago switched to push fit connectors, which are used for both types of piping. They are commonly known by their trademarked name: Shark Bites.
They are far more expensive than ordinary copper connectors, but this is offset by massive savings in the cost of installation. A 4 hour job soldering copper pipe is reduced to about an hour in my experience. Mistakes are easily corrected. They are easier in so many ways and as a bonus you won’t set the house on fire.

This removes one of the chief annoyances of using copper piping. Soldering is time consuming, exacting and it is very difficult to fix mistakes when they happen. This same connector can be used for PEX, but I’ve had them leak during installation. That never happens when installing these on copper when installed properly.

The typical and less expensive PEX connection is done this way:
the PEX is first flared with this tool so that it will fit over the connector:
and then crimped with this one.
You can also use a gauge to make sure that the crimp was done properly.

They are kind of a pain to remove vs. a Shark Bite, but much less expensive and less likely to leak.

So on to the comparisons:

The rigidity of copper piping makes it hard to run in 2x4 walls. You have to do it in short segments, so nobody does this except when they absolutely positively have to. More joints means more possibility of leaks. You go under the house or in the attic. This is a better approach because it means you’re not going to accidentally poke a water line when you putting nails and screws in a wall, but sometimes it just has to be done because there is either no other way or if there is, it involves damaging something along the way that you’d much rather leave alone.PEX Pipe Fittings manufactures

In this case PEX is a great option. It’s flexible, requires fewer joints and you can thread it in a wall. It can come up through the floor and curve around rather than using joints. It is particularly useful when you have odd angles that have to meet. Doing this by soldering with a couple of 45 deg. elbows is quite difficult.

PEX expands, making it resistant to cracks from freezing, it does not corrode, it does not water hammer and can be connected to existing plumbing with the right connectors. Also they’re color coded, so you don’t have to guess which line is the hot water.

However it is not suitable for outdoor use because UV light will destroy it over time. It can’t be recycled at this time and it requires a different set of tools and connectors. Also, like all other plastics, it can be damaged easily through abrasion, although this is highly unlikely in nearly all installations.

It’s my understanding that it’s catching on in new construction, but I don’t see much of it being used for repair or remodels. I can be wrong on this point. Those are just my personal observations.