I do too much aluminum and stainless to justify adding a mag-ground to my huge pile of miscellaneous crap. I've used them in the past, and find them very practical for large fabrications and work around sensitive equipment where a ground should be as close as possible to the work.
Since I do so much non-magnetic material, I've made a ground out of 6' of stripped 1-0 welding cable made into a "noose", that I can cow-hitch around a pipe on stands for a free-rotating ground, or simply throw in a heap on whatever I'm welding. Still, I don't use that one much, as I so often have a stud or bracket nearby to clamp my regular brass ground to. That's the nature of my work, though. If I went back to building railcars, I'd want the mag-ground.