I use a SPOT I. It drops electronic bread crumbs every 10 minutes that a trusted friend can watch at home. It also sends and OK signal, a HELP signal, and a 911 call. These can be programmed to go to email or as a text message to a cell phone. We use Help for a non-medical emergency where it's not critical to be picked up immediately.

At the beginning and end of each day, we send an OK signal, although he knows we could be in a dead spot.

From reviews on motorcycle forums, I've learned SPOT I has better coverage, seems to be more reliable, and battery life is longer (about a year with daily use.)

I've never activated it, but I consider it one of the essentials. If I need picked up at some random road along a trail, it makes it easy to send coordinates to the person who will pick me up. Then they can just get directions from Google.