Now, for any job there are lots of tools that will work, and usually one or two that are perfect for the task. In this case, I decided on getting a reciprocating saw because I thought it would do the job, and also it's extremely versatile and would find lots of uses later on. Some quick Googling revealed an oxy-fueled torch would probably be better, but I'd probably never need it again.
Renting a reciprocating saw (commonly referred to as a sawzall, even though that's Milwaukee's particular model name) would cost me about $20 a day. Not bad, but since you can get a decent one for around $100, I figured that made more sense (if I use it five times in the next few years, it'll be worth it, and I have no doubt that I will).
Next; research models online. Almost everything I read pointed me in the direction of a Milwaukee brand saw (they're the original, after all). And this write up at Popular Mechanics sealed the deal. I picked up the Milwaukee 12amp Sawzall and a diamond-coated blade, and went to work.
Check out the video below to see how I did (note that in the video I'm calling the material cast iron, but I later figured out that I think it's actually galvanized steel, so I may have had the wrong tool for the job).
This isn't exactly a straight-up review, since I can't compare to other brands/models (and there are tons of those out there already). Overall I was impressed by the quality and construction of the Milwaukee saw, and it seemed to have plenty of power. But I was a little disappointed that I couldn't cut through the pipes a little easier, and my blade wore out (and then broke) long before the 30 uses it claims on the packaging. That could be because I didn't get the right blade to use on steel, but I still would have expected better.
I'm going to hold off on a final judgement on this one and wait until I get some more uses in on other materials. When that happens, I'll check back in and let you know how it goes!