I assume you are talking about magnetic signs on cars to promote your business....? If so, I swear by them. I have picked up quite a few customers who see my signs. In fact, one time I was in a store and my cell phone rang. It was a lady who saw my car signs. She was IN the store, so met up and she ended up being a really good customer. I have seen a couple people take photos of my signs so they capture my phone number.
There is only one drawback: if you live in a tropical climate like I do, always removed the signs after a couple days. They can adhere to your car doors making them very damaging to remove.