Back in 2005, when Smart cars started "wearing" advertisement, I worked for a company called Noheto. It was a content management software. I used to say that choosing a content management system is like choosing a mean of transportation. You want to get it right, especially in the long term. So we used this metaphor and compared our product to a smart car.
You should have seen the desire in the eye of the people we talked to. They all wanted it. We would make it very explicit that we only had a few, which would only make it worse. :-)
We'd put inside a short description of the product, the card also contained contact info.
Even the competitors would come and ask us to have one at the end of the day. One of my client said, I give your goodie to my son, he took it so fast from my hands, it was amazing. I saw it for years among his toys afterwards.