No use on alloy or some stainless unless you put a steel plate/another magnet behind.
Can't weld close to them.
Pick up loads of dust/dirt/slag/scale and can be a nightmare to store.
On the other hand, they are quick and sometimes there are places where a clamp won't go.
I don't like the adjustable type as you need 6 hands to get them in place but I have a few of the corner type squares that get a fair bit of use.
They are handy for fitting into jigs as well for quick insertion/removal of parts but that's only useful if you want to make many of the same thing.
I have a few dti gauge bases that I have doubled up to make adjustable clamps and they work ok for holding sheets together and stuff but I could live without them I think.
If you are butt welding an arch repair panel in and you have no access to the back then they are very handy....half a dozen keep it in place and allow you to tweak it....then tack it in and remove the magnets. Means your 2 pieces are perfectly aligned...not sure how else you'd do it to be honest...most surface clamps have a pin in the back?
Same for sill repairs...clamp the flange...but if you are butt welding it in then I use the magnets to hold it in alignment until its tacked in.
No good for making gates...but they have their uses. Also handy for holding up temporary spray sheets if you have a metal frame roof.