It seemed like a neat idea, but after using them consistently for a month to try to see how useful they would be, they strike me as a solution in search of a problem. These really only make sense if you only ever plan on using ONE cable with your device, and you don't want to use that cable with any other device. I could see this being useful to get a little more life out of a phone with a port that's started going bad, perhaps, but outside of that circumstance, they turn out to be a wasted product.
Sure, it seems convenient to just grab your phone and walk off without even having to unplug, and having the magnet just release, and it does work as designed in that respect (assuming it doesn't run into problems like many others on here). But, then that cable is useless to you if you want to charge any other devices. I like to keep my cables busy. I unplug one device, there's a good chance I'll want to plug another one in that I'm not using at that moment, especially with modern fast charge technology. That's a much harder thing to do with these cables. More, if your phone still has that little tiny bit plugged into it, you now can't plug it into another cable without pulling it out, and that's actually fairly difficult to do. It's not really designed to get a good grip on it. If you're older, and don't have nimble fingers or good grip strength, or if you have large fingers, you might require pliers.
I'll save mine for if I end up with a phone that has a port that's worn down some and doesn't like plugs going in and out. That's really less of an issue with USB-C though, so largely, I think this product ends up being totally useless under most circumstances.