I haven't plumbed up my shop yet for compressed air and PEX is my leading candidate. Sure, I've stared long and hard at the Rapidair systems, and they make a nice kit with good street cred and look fantastic, but let's be really honest here, when you have a large shop, with a lot of ground to cover, and want to make a lot of custom drops for the assortment of workstations and tools, those kits just only start to cover what you need. I've got a 1300 sqft shop / garage and plumbing it up the way I would want to using Rapidair would approach $700+ w/ the need for several base kits and additional fittings / manifolds, and some larger pipe stuff as well. This is where I (and others) turn to considering PEX. The base materials are vastly higher volume therefore lower cost, higher availability, more configurations available, which gives us DIYers exactly what we are looking for. We can build and design the system on the fly, make a few trips to the stores along the way, and have a working system up and running over the weekend. Most of us aren't pros. The way I see it that kind of thing is what this whole hobby is all about!
Now, copper for me is also a consideration, and has most of the same things as PEX with a bit more skill and planning required (such as having to nail the bends down perfect before mounting / installation, whereas PEX can be formed and installed in place and trimmed as you go). That translates to likely more time and frustration to install a copper system than PEX, as well as better planning required (not everyone's strong suit), which coupled with the higher material costs of the tubing, is a strong consideration.
Just my take on things. I've plumbed up nearly my entire house w/ PEX rather successfully. Learned a few lessons along the way, picked up a few custom tools to make the job go smoothly, for compressed air in a hobby shop, from my persepective, it's a very good and viable option. I'll definitely be including it in my never-ending shop build thread once the time comes (next 6 mo or so).
