In order to determine if Cross-Linked Polyethylene (PEX) it permitted in your jurisdiction you would need to call your local Plumbing Inspectors office.
Although I do feel that PEX is a quality product I am not nearly as convinced that it is the "Magic Bullet" as some would have us believe.
It is true the PEX tubing will tolerate an occasional freeze but it must also be remembered that all the fittings and end termination stubouts are made of brass or copper, and they are subject to the same types of freeze damage as we might see in copper. In addition, for reasons known only to the critters involved, rats, mice, squirrels and opossums seem to have a sweet tooth for PEX tubing.
I agree that the initial cost of the PEX tubing is much less than copper however if you elect to run a PEX manifold system you will use so many additional feet of tubing that in the end the price works out about the same as a copper installation. In fact many plumbers now bid copper or a PEX manifold system for the same price.
It is argued that PEX has the advantage in reworking an existing structure because it can be fished through the walls like pulling an electrical cable rather than opening the wall to run a rigid pipe but on the other hand, the same thing can be done with rolled copper pipe.
As for installation. As was mentioned in the previous post, PEX uses a crimp on system to make all connections. It must be noted that there are two or three different types of crimp fittings which are approved and you must be very careful to insure the that crimping tool you select is listed as approved for the type of fittings you intend to use. There is also a Go/no go gage that is used to check each crimp to insure it is set correctly. The use of the gage is not an option, you are required to have it and use it on each fitting.
It should also be mentioned that there are some compression fittings that will fit on PEX tubing. Those fittings are approved for use in the HVAC industry on low pressure hydronic heating lines but they are strictly prohibited for the house potable water distribution system.
There is a little known characteristic about PEX that often goes totally unnoticed by DIY'ers and sadly some plumbers and inspectors. PEX may not be exposed to ultra-violet light or direct or indirect sunlight for a prolonged period. When a PEX manifold is installed in a basement or when running PEX tubing through joist bays in a basement open crawlspace the tubing must be sleeved with an approved UV protector or the joist bays must be covered to prevent sunlight from passing through a basement window or into the crawlspace and being reflected to the PEX location.
My personal singular objection to PEX is a health issue. Under laboratory tests it was determined that Biofilm will form the quickest in new copper pipe, followed immediately by PEX, however, as copper ages it forms a molecular layer of copper sulfate on the interior of the pipe, which is often seen as a greenish corrosion layer. Copper sulfate is one of the best anti-bacterial agents known to man, so in essence, by the time that Biofilm has had a chance to begin forming in the pipes copper pipe forms its own antidote. On the other hand, it is recommended that a PEX or CPVC system must be sanitized at least once every three years. Biofilm is a gelatinous mass that forms in pipes and AC condensate trays which has been directly attributed to Legionaires Disease and a number of other potentially deadly pathogens.
If you would like a copy of the PEX installation handbook you can download it free in PDF format from the following link.
Plumber's Installation Handbook