I don't have one of these dudes, but others refer to them all the time for use in cutting off excess brass when forming obsolete cases from common cases, e.g., 12mm Swedish from blown-out .348. Are these just cheap little plumbing tubing cutters? What do you guys use?

IIRC from building a house, there are cheap tube cutters and heavier, bigger ones that cost more that cut faster and neater.

Yes, I used a quality tubing cutter (steel line cutter actually, but they are pretty much the same tool) from Snap-On to trim a barrel. It worked very well. - Dan

I have the cheap $10 one from Home Depot and it works just fine. Remember that most cases are tapered, so if you turn the cutter in one direction then you will start threading the outside of the case. So turn one way, then the other and make sure the cutter wheel is rideing in the same groove. Cut a little long and then trim to length.

I prefer a small lathe with a cut off bit. It is quicker, cleaner and more accurate. After the cut and powering down I use a countersink for the inside de-burr, and carefully use a small file to clean up the od burr.