Yup i have one! I've had it since i was about 15, and they work too! Mine fell off once in high school and i didn't notice till well after the fact, and a few days later i got called to the office saying they found it. Here in Canada, MedicAlerts DON'T have your name on it; just your medical information and then you have an ID number on the bottom, and telephone hotline number at the top.
The person called the hotline number and gave my ID number and that's how they found out who i was. I'm glad i found out they work through that instead of an acutal medical emergency! Get one if you don't have one!