Another SPOT user here, 4 years now. I mostly use mine to communicate with the family/wife and send in OK messages. Never had to use it for Emergencies yet, but two of my friends have used it. Within 4 hours of the SOS signal, the S&R were climbing down into the slot canyon where they were stuck.
I usually keep my SPOT II on my PFD the whole time, in case I get thrown out of my boat. I send the OK signal before the rapids and after we get to camp, so my wife knows I am ok.
One time on Cat after all the Big Drops, I forgot to send in the signal. My wife freaked out and got all my rafting buddies riled up to go S&R. Luckily I remembered the next day and sent a signal.
PS: $100/year is a small price to pay for family comfort that you are ok.