D1 has one of my cards for emergencies, and for things I will pay for (for example, when she is home and picking her sister up from school every day 20 minutes from home, I will tell her to use it to put gas in her car). She would never use it for something I have not specifically allowed.
One semester when she was not on the meal plan for school I gave her a dollar amount she could spend on food. If she was frugal at the grocery, she could eat out cheaply a couple of times a week. She did that using my card, but she was careful not to go over the budget we had discussed.
She just graduated from college and is moving out to another city for her job in a few weeks. She is going to hang onto my card, but as an extra card to keep in a folder in her file cabinet in case her purse/card (her own) is ever stolen or lost. We have a deal that she will let me know and reimburse me for any expenditures from it in that case.
I will say that because she is a responsible kid, it became MUCH easier to handle accounting/etc. for things I will cover for her when I gave her a card. I didn't have to pay her for stuff I said I would cover, etc.