We use them at the inn where I work. We also have them in our gift shop. They are quite a good seller. I think that when people use them, they want them. We have a couple of other things in our gift shop that we don't use on our tables and no one ever buys the items.
Months ago, there was one lady who used a mug for her cofffee one Sunday morn. She loved it.
She says to me, "I'd like to buy this mug."
Me: "We have a few other colors on the gift shop shelves, if you'd like to look. That way you can buy a brand new mug."
Her: "No thanks, I want THIS mug."
Me: "Ok, I can do that. Would you like for me to wash it for you before you leave?"
Her: "Nope, just put the total on my bill and I'm going to drink out of it on my way home!"
Me, after I walk back into the kitchen where she can't see/hear me: "Okay lady, whatever floats your boat!"