I don't leave home without it! But I have forgot to get it off the tractor after loading up on the trailer. I usually figure it out when I reach out to get a drink in the truck and there is nothing there. Do a quick pull over, hop up on the trailer and the cup is still there.
So not only is it great for holding ice the magnetic cup is also saved me a few bucks by not loosing them either. I have wondered what other drivers have thought when they saw my big cup holding fast to my tractor fender going 50mph down the road.
Heck this cup could hold a thread of it's own.
Gordon