... How strong are the magnets? Will you notice if you snag on something and pull one away? ...
They are very strong and hold tight. You have to pull pretty hard to get it loose. And yes, I would notice, though I am very situationally aware, am not doing penetration or cave dives. I don't even do external wreck dives - not interested. Exactly what do you think I'm going to snag on? The coral? Not possible. I stay at least 5-10' away from anything solid . A passing fish maybe?
... Do other things get stuck on them, like a d-ring, bolt snap, or even your own steel tank? ...
They don't do that with d-rings (SS so not magnetic), bolt snap (brass or SS) and I can't imagine them attaching to my tank as I would never just release it to hang free. It's either in my hand or clipped.
... IMHO, a SPG should never be dragging, and a magnetic QD could allow it to happen. ...
Agree nothing should be more than a few inches from your body. Nothing should be dragging. Could allow it to pull free, maybe, but
really unlikely, and not without me noticing. And again, on what?
... I've seen octo clips and mouthpiece holders come loose all the time. ...
Yep. I've had that happen to me. Hose clips are useless, octo holders that I have tried, the same. They come loose all too easily, are difficult to reattach properly, making it easier to think it's connected when it's not. These QR magnetic holders, no. I've never had one come loose. I've never had a problem reattaching. They snap into place hard and you know they are set.