I have a SPOT I which is supposed to be more reliable than a SPOT II. It sends out my position every 10 minutes if I remember to turn it on. Here
is a track from my last trip.
I program the OK button to send an OK SMS message as well as an email at the start and the end of the day or if we stop for a long time during the day.
I use the "Help" button to request non-life threatening help. If it's at a road, it's simply a request to be picked up. If it's on a trail, it's a non-medical problem. For instance we are lost, but can hold out a few days while we wait for help.
It can also call 911 which is what it sounds like. That goes directly through SPOT.
Yes, it's expensive. I think $99 for the unit and $150 a year. But I consider it essential gear in my pack. I've found the batteries last about a year with everyday use for a few hours. That makes it better than a cell phone.
I'd suggest attaching it to your pack with a cord as they are easy to forget.