I have a spot messenger, haven't had to use the SOS function on it yet (the epirb part of its functions) but the pre set messages come in real handy to let those in a responsible position to know you made it to each destination safely.
There is also a HELP function, that tells pre set people you need help, but not rescue, say, made an outlanding (precautionary landing) etc, and just need to be picked up, need fuel or whatever. Something that doesn't require the emergency services.
Fits nicely in the top pocket of the flying jacket. Also, for a subscription fee, it offers real time tracking to a website. So interested people can follow your progress. Though this goes through a web server, as does the help messages, the message is passed on to SMS phone messages or email. EPIRB SOS function goes through a manned emergency response centre in the USA, which then pass the distress message and gps location on to Australian authorities.
Not only good for flying, but motorcycle touring, kayak trips, etc etc