Update:
Well this experiment paid off well. The pic is of 108 caps for wooden posts, made of .25" CorTen steel. Lots of grinding on these as the customer wanted a #1 finish on the corners i.e. ground smooth until no trace of weld remains. There are supposed to be a few hundred more of these coming, so this mag chuck is going to be used more and more. If anyone is thinking about buying a used one keep in mind the lever that moves the magnet can get real sticky. Of the three I tried out at the used tool dealer, only one was easy to actuate. I ended up dumping some Starrett M1 lube inside of it, and it moves pretty good now.
In actual usage, I never even actuated the lever, I just left it on and set and removed each plate as needed. After a few parts, I figured out where to set the plate to find the sweet spot between ample holding power and ease of removal, but for 6" x 6" pieces of .25" material, it was a breeze. I will never ever go back to the bench vise for this type of work, its that good.