I JUST ACTIVATED THIS ON MY iPhone 6+ and it works great. Essentially, it allows access to contact data and medical data you choose to share to anyone using your phone to try to help you in time of a crisis:
With all the hoopla around Apple’s release of the iPhone 6 and iOS 8, it’s been easy to skip past some things. For one, Apple delayed the release of apps that connect to its new HealthKit functionality in order to fix some last-minute bugs. But the Health app itself is there on iPhones running iOS 8 and if you take a few minutes to configure it, the results could someday be a matter of life and death. Unlike all the pronouncements calling the new iPhone the “best ever,” that statement comes without a trace of hyperbole.
Medical ID: Even on protected phones Many of you probably have a contact in your phone labeled ICE, for in case of emergency. It has the name and number of a spouse, parent or close friend who you’d hope someone will call if, heaven forbid, you’re seriously injured or fall ill. The problem with that ICE contact, of course, is that if your phone is locked with a passcode or your fingerprint, it might not be possible for someone to access your contact list after you’ve become incapacitated. In theory someone could take your phone to your unconscious finger and try to unlock it that way, but with iOS there’s a less macabre solution. And if you haven’t already, you should set it up right now. (Thanks to 9to5 Mac for drawing attention to this.)
The process is quite simple.
1) Click on the Health app and then choose Medical ID from the lower-right-hand corner.
2) The first time you do this, you’ll get a descriptive screen about Medical ID and a button to Create Medical ID. On future visits to the Medical ID page, you’ll se an Edit button in the upper right...
I can see this being a critical feature in a time of crisis. I like it.
One more thing for iOS 8 users: Don't activate iCloud Drive: It causes some problems.