At one time or another Iíve had shops set up with PVC, Iron and copper.
PVC, as many others have noted, can be very dangerous; the pipe is not designed to be used with air and has integrity issues with changes in temperature and any exposure to sunlight.
Iron was safe and secure but had several issues: it was a pain to install because we had to thread almost every joint and after about six months started to become a maintenance nightmare as the rust in the lines started causing issues with my machines. Even just using the blow guns was messy with all of the rusty spray going everywhere.
Copper is what I had in my last shop and overall was a good system; secure, safe and while not maintenance free (there were some electrolysis issues) it was a solid system. The only negative things about it were: it was very expensive! That copperís not cheap and while easier than iron to install it still took a long time to sweat all the joints.
I built a new, and hopefully last, shop a couple years ago and used a new aluminum pipe system that i wish had been available back when i did my first shop. It has all the "pros" and none of the "cons" of every other system Iíve used. It is safe and secure, extremely easy to install (it took a helper and myself less than a day to install air completely throughout my 8000sqft shop), to date Iíve not had any maintenance or contamination issues, and best of all it was inexpensive - especially compared to copper!
If you want to see any pictures of my latest (last, if I have anything to say about it) shop let me know and Iíll shoot you a couple. Here is the place where I purchased my system http://www.speedsourceusa.com/prevost-air-systems.html
- They were easy to work with and helped me out with the design layout a lot.
So to boil it all down Iíd say no to PVC and Iron and yes to either Copper or Aluminum.