BillConner wrote on Jan 15^th, 2013 at 2:57pm:

Do you know of an instance of a Spot having failed to send an SOS message?

Check out the stories in the GPS City link I posted earlier. Although the "SOS" and "I'm OK" buttons work a bit differently in their attempts to send messages, I suspect that in practice the two have close to the same reliability. You will have to decide for yourself whether many peoples' problems result from not reading the directions. I assume you would test your unit out for mechanical soundness before going into the wilderness.

My SPOT unit worked fine under normal (WCPP) tree cover, and also from inside a tent. I was encouraged by the backpacking mag's tests in Utah slot canyons too (see other link above). So the "clear unobstructed view of the satellites" requirement may not be as bad as it sounds.

With a SPOT unit, you do still have to press the buttons. If you are incapacitated under an avalanche for example, a PLB will help you, where a SPOT will not.