Friend of the site Stephen Hackett goes over the benefits of setting up Medical ID in the new Health app on iOS 8. My wife and I also set this up on our new iPhones almost right away.
I, for one, am very happy to see Apple offer an easy-to-access solution such as this for something that is so important. And because it is now available on every iPhone running iOS 8, I would guess it won't take long for many medical personnel to become familiar with Medical ID's existence.