I have a lot of love for Pex pipe and the benefits it provides (freeze-thaw resistance, high pressure, flexibility). However, I asked my father, a plumber, this same question about using Pex for compressed air, and he had a fairly substantial warning. Standard Pex pipe is designed to be buried in a wall, with no exposure to UV light. I was warned that unless otherwise noted in the specs, most Pex pipe is going to degrade with exposure to UV. Virtually every compressed air piping system I have seen has some point in the setup that is exposed.
I am pretty skeptical about the Sharkbite fitting system. I prefer the Wirsbo fittings because the theory behind them makes a million times more sense to me, with little or no risk of failure. The crimping collar is made of Pex and has a memory. The expander tool warms and stretches the fitting to a slightly larger diameter than the pipe. When the collar is slipped over the pipe at the fitting location, as it cools it attempts to return to it's original (undersize) diameter. This means that the fitting is under constant pressure. Unless the fixture is reheated, the pressure will exist beyond your lifetime. The tool isn't cheap (about $400), but some places have them for rent. The collars are about thirty-five cents apiece. Between the tool rental and the collar cost, that's cheaper than my time attempting to solder copper. Others may find the price/performance balance of copper to be more in their favor.
There are good points above about drainage that apply to any system of any pipe type. I also agree about filtration, although I'm always willing to invest in the ounce of prevention. The last system I set up from scratch was copper (customer choice) but I added the Dynabrade FRL units to each shop outlet. This system was really designed for sanding, though. In another shop, where the air was also supplying a CNC, we went with a separate appliance to dry the air. Cheap insurance.
