PEX stands for cross-linked polyethylene, and it has been used in commercial applications for years. If you have ever been to a fast rood restaurant or movie theater, and you ordered a carbonated drink, the ingredients for that soda came through PEX tubing to the fountain head where they were mixed together. This product was first used in low cost housing, because it was discovered to be a cheap alternative to other piping means, but it has been finding its way into higher priced homes. I am not sure how cheap it might now be with rising petroleum costs, since this product is derived from the petrochemical industry, but it is still cheaper to install than other piping means.
Maybe I should not have used the word cheap above, but rather cost effective. The product is actually quite good, so I do not want you to have the wrong idea about it. This tubing can run from the water source to the fixture without any joints/couplings. This means that the possibility of leaks decreases, since most leaks occur where two pieces of pipe join together. The system works by running all cold and hot water through a manifold. At the manifold, you can turn off the water to any individual fixture or appliance. This is nice, because the valves for showers or tubs cannot always be accessed. From the manifold, there should be one piece of tubing all the way to your fixture, like a sink. Red tubing is used for hot water, and blue tubing for cold water, but there is absolutely no difference in the make up of the tubing. This means that hot water could run through blue tubing and not harm it. Manifolds are typically located in the garage, and each valve will be labelled as to where the water is going.
What problems may you have with PEX? I am a fan, since I found this tubing easy to work with in commercial settings, but in my home inspections, I have found out about some issues. I have never seen this yet, but a builder reported to me that the main call back that they have regarding this system is a leaking manifold. This may be due to damage done to this device during construction, or homeowners over turning the valve, so you should check the manifold every so often during the first year at least to check for leaks. On one inspection, a plumber had run out of red tubing, so he used blue tubing for the hot water for one sink. By not staying consistent, someone can be confused in the future. In a commercial setting, I saw where a tube was not long enough, so the product had a joint made in it. Some tugging on the tube by a repairman years later caused the joint to open up causing a leak. I have not seen this in a residential setting yet, but you never know.
I think that PEX is easier to work with than PVC. It is surely easier to carry a roll of PEX than several tubes of PVC, along with its couplings. No special glues either. As builders of homes at any price range look for ways to improve their costs, PEX just may become a standard, which is not a bad thing entirely. I wish that I had a manifold for the water in my house to make those weekend plumbing projects simpler.