gas cylinders repurpose?

In the HVAC trades we use them pretty much like nitrogen..... Personally, I like it better. Because its liquified, you have much more volume per tank. For pressure testing its stable, like nitrogen, and the extra volume makes it great for blowing out linesets etc.
Some gas shops will exchange the bottles and not care about the pepsi on it. Others wont....
IMO the alum. bottles are better then the steel. When you exchange, try to keep alum. if possible. Its lighter in weight.
One last thing, You can get a regulator to screw on the bottle that allows you to hook up anything you want, ie: 1/4 in flare for hose to blow gun, or hvac guages etc.