I wear a dog tag (yes, a real bow wow dog tag) on a lanyard around my neck with my name, next of kin, insurance number and the name of the hospital I work at engraved on it. I keep it on a lanyard around my neck with my keys.
The best deal on them can be had
here. I got mine in red plastic. Less rattling.
Yes, they are useful. ER nurses look for them on unconcious people brought in. Shoes tend to get thrown off in accidents or tossed in the corner of a trauma suite. Things around the neck are noticed. You want the ER staff to have the information. First responders typically cut off helmets, and sometimes leave them behind. I would avoid having the tag attached to a helmet.
You can also place your next of kin's phone number in your cell phone under ICE, or In Case of Emergency. That is checked as well.