I find it interesting that the thing SPOT users mention as a benefit is that they can use the SPOT in essentially the opposite way as a PLB.

With a PLB, nothing is heard from the user until rescue is needed, and then the device is turned on and heard and provides a fairly exact location.

With a SPOT, they can press the Help button when needed - kind of like a PLB - if the Help message is received along with a fairly exact location, gets to the right people, and rescue is hopefully arranged.

This is the first comparison I'd be interested in seeing ... what is the successful rescue rate for PLB vs. SPOT-like devices?

BUT - from comments given in forums - their view is that the SPOT can also be used in an opposite way. The user communicates an OK message on a regular basis, and it is pre-arranged that the ABSENCE of an OK communication signal indicates that rescue is needed. Supposedly the family/friend can use the previous location reports and the trip plan to estimate the user's location. This makes me wonder about how large that search area could be if backpacking, or canoeing (ala BWCA).

Of course the problem with this later method is that a lost or malfunctioning SPOT can result in a false rescue attempt.

This creates the second comparison I'd be interested in. I wonder how often what I'd call "no-signal rescues" happen for SPOT-like devices? AND I wonder how many of those rescues are not really needed?

Ken