EDIT: Apparently the service didn't actually launch until today, despite Apple advertising it for several months. Keeping comment as originally posted for posterity. However, if this is as half-assed as Apple Crash Detection, expect it to work about 1/10th the time.

A few weeks ago had experience with this up in the Mt Baldy area of Los Angeles.

During inclement weather, a hiker in another group slipped on the Devil's Backbone and tried to use Apple's Emergency SOS because their phone did not have cell signal. It didn't work.

We ended up letting the Mt Baldy staff know about the hiker when we got to the Baldy Lodge. We asked them whether emergency services had received the SOS, and they said it hadn't. And they would know, since emergency services would have contacted them to conduct the rescue.

Long story short: Don't trust SOS. It doesn't work when you actually need it to work. And when it does work, you could have just called 911 directly.