First off, I have no dog in the fight and use PEX, CPVC and PVC according to what is most suitable for the application. I would like to correct and expand upon some of the statements made in a previous reply.
PEX is cross-linked polyethylene for use in both hot and cold applications.
PE is not cross-linked and is acceptable only for cold applications.
There is no appreciable off-gassing difference between CPVC and PEX in hot water applications as all non-metalic pipe and tube must conform to ANSI/NSF 61
CPVC is sized according to CTS compatibility thus 1/2″ CPVC is essentially equivalent in flow rating to 1/2″ Copper.
PVC and CPVC are joined with a solvent cement, not a glue. To be done properly, the joints must be mechanically and chemically cleaned as necessary. Then primer is applied followed immediately by the solvent cement which creates a chemical weld of the pipe to fitting. The majority of solvent joint failures are caused by human error. PVC and CPC can also be thermally welded using special equipment and techniques.
PEX fittings have a smaller bore diameter, thus up-sizing is required in PEX systems where flow and friction losses must be addressed.
The expected use lifespan of PVC and CPVC is 50–75 years, not 20. Likewise the accepted lifespan of PEX is 50 years in cold water applications and 30–40 years in hydronic applications.
PVC/CPVC freeze much slower than Cu and PEX. The Wm/K values are as follows:
PEX - 0.51
CPVC - 0.14
PVC - 0.19
Having observed freeze condition in Fe, Cu, PVC, CPVC and PEX in actual applications, there’s little difference in the results except in very rapid freeze conditions where PVC and CPVC can splinter rather than just split. Typically Fe, Cu, PE and PEX will open a single rupture running parallel to the bore. PVC and CPVC will rupture randomly. The proximity of metallic fittings will greatly increase the likelihood of freeze-rupture on all non-metallic pipe and tubing whether PEX, PE, PVC or CPVC.