You can either get a bracelet or a neck pendant which I have, which is a little less obvious, but also useful in a real emergency.
If you suffer allergic reactions then these can be very severe and the bracelet/necklace gives a freephone number which can be used worldwide for doctors/hospitals to get your confidential records listing what you are allergic to.
If you are unconscious It is obviously better to know before they give you any medication what you can and cannot handle. It costs about £15 a year for registration and I guess it's like any other form of insurance, if you don't need it then it is money wasted, but on the other hand it could save your life!