When I researched Rapid Air, I found the 1/2" systems are inexpensive, but I wanted 3/4". The tubing for the system is comparable to copper, but all of the fittings are massively expensive. For example, every 3/4" tee fitting is $20 and each wall-mount outlet is $30. Compare that to like $3 for a copper tee and < $10 for each wall outlet (I used a tee, a 3/4" NPT adapter, and a 1/4" reducer for each outlet). I ruled Rapid Air out because of expense. If you don't need many outlets or don't mind spending the money, these systems seem nice.
I ultimately went with copper over iron pipe because I find soldering copper to be simple, and I found a deal on a bunch of 20' or 24' sticks on Craigslist. I didn't look too much into iron pipe, and am not very familiar with how to join sections and fittings, other than threading - and I didn't want to buy threading tools or have to deal with buying different lengths of pipe.
Things I learned or would do differently:
1) use loctite 567 pipe thread sealant on any threaded connections, not teflon tape
2) i put drain pipes under all of my outlets - in hindsight, this was overkill for my setup (my drops come out of the top of my pipe and my compressor has an aftercooler) - I've never had a drop of water come out of them
3) don't buy cheap ball valves. get good, full-port ball valves where air will be flowing through them to your tools (vs just the drain valves)
4) it would be best if your flexible connections have at least one swivel end. I used hydraulic hose with no swivels to attach my compressor to the air system and i threaded everything together as I built the system. I can't pull the hose out of there (to replace one of my leaky ball valves) without cutting it out. oh well, good thing that leak is really small
5) get the Milton V couplers and plugs, like Schurkey said. there are sellers on the sears website that have the couplers for ~$5 and plugs for < $1. I think I spent less on these fittings than the more restrictive ones I bought at Lowes.
