I'm hearing pros and cons on this on both the Diabetes Talkfest and Diabetic Rockstars communities, the deal being that most first responders (EMT-A or EMT/basic) will not be trained to deal with diabetes, so knowing you have diabetes is not going to affect how they do or do not treat you, other than maybe to get a doctor on the radio sooner rather than later. They are not trained to administer a blood glucose test or to give a glucagon injection or IV.
Some community members also said that EMT-As may be taught to ONLY check the LEFT WRIST for a medical alert ID... so necklaces and wallet cards might be less effective than bracelets.
If you have multiple medical issues, or are extremely brittle, an alert bracelet with a subscription to the MedicAlert system may be a worthwhile investment. Also, training a couple of friends and colleagues to recognize general highs/lows and basic remediation (water, OJ, glucose tablets, and if you find yourself in need of it, a glucagon kit).
Personally, I usually rely on people around me knowing that I have diabetes and what behaviors might signal that I'm "off". I carry my blood glucose monitor like a personal talisman, and I've just put a T2 wallpaper on my cellphone.