I have and use a SPOT. I've never used it in a real 911 emergency as of yet. But do use the tracking feature which drops a way point every 10 minutes along a path/route as I'm moving. Using what SPOT calls a SPOT Adventure web page, others can track your movements in near real time. Once I stop for the day, I turn off the tracking function and press the OK button to notify friends and family that I've arrived at my destination for the night and all is well.
IMO what makes me feel safe is the tracking function, For example; I was following a buddy of mine as he crossed into Canada on his way to Alaska using his SPOT adventure page. At one point I noticed way points being dropped along the side of the road. I called him up via cell phone to find out what was up, his motorcycle engine was blown and he was waiting for a tow. He was quite surprised by the call.
Keep in mind regarding a PLB, Delorme's iReach or even the SPOT, is that your still at the mercy of the rescue infrastructure that's in place in the area in which your in. Once that button is pressed, don't expect a squad of Marines in a Blackhawk to suddenly show up. In places like Central and South America, the person who shows up first may be a village elder on a donkey.