Never really bothered with them, they pick up all the crap from the grinder/swarf/bits of metal etc that sticks to it in a massive clump. Everytime you touch it you get metal filings stuck in your fingers. IMO, not a worthwhile replacement for a good strong earth clamp.
Ah yes, i remember why i bought some - bolted a couple of halogen lights to them, and a couple for the nederman extract hood so i can stick them onto whatever i am working on. That's one thing they are good for!
Aye, now they are a waste of time. Can't beat an angle plate with clamps, or a big bit of box section - tack it up and it aint going anywhere while you weld it!