Spot system works if you understand that you need to keep it on for a while. It sends out a signal every 5 minutes, and some of them don't make it out -- I think it is more often that a satellite isn't overhead if you're in a canyon. If you're on a peak, every signal gets out.
If you use the "send help" signal, a transmission goes out every 4 minutes. I don't know how often the SOS signal goes, but I am sure it is sent at least that often. In the Sierra, the biggest problem is when you're in a canyon or have tree coverage.
I wouldn't hesitate to use Spot on any trip, though I'm planning on stepping up to InReach next year due to it's two-way texting features.
