I thought about one of these, but there was a thread either here or on BR discussing it, with input from people who work in the emergency services.
What it seemed to boil down to was that if you're fecked up enough not to be able to give your name, then anyone medical treating you isn't going to arse around with pendants, wristbands or phoning your partner to tell them you'll be late fro tea. they're too busy saving your life (and they have to test and match your blood before giving a transfusion anyway, so blood group is redundant information)
After that, it's the job of the Police to find/inform next of kin, and they are very good at that, especially if they've got a name to go on.
If you don't have any other kind of ID that you carry, then a wristband is probably worth it to speed up the Police's job.
Actually I might get one.