Mine works great. Take it with me every trip.
You only have 3 messages, "I'm OK", "I'm in trouble", "911". You can pre-program the messages. But, it's not a blackberry, so you can't send a text message. There's also a tracking mode.
It has a GPS chip in it. When you send a message, it tacks your GPS coordinates onto your message, and beams it up to a Global Start Satellite. Those satellites are the same satellites you'd use to make a satellite phone call. There's Global Star and there's Iridium, this system uses Global Star. It's a text message so doesn't need a very good signal to work properly. Not like a phone, which needs a very good signal. Once you send a message, it either sends the message and coordinates to an Email list you created, or goes directly to the 911 center. All 911 can do, is tell the local sheriff, your requesting help, and give the GPS coordinates.
Now, the SPOT device needs to be able to see the satellites. The SPOT LOGO on the the unit needs to point at the satellite. If your on the North side of a big mountain (such as I am most of the time) you will be lucky to get a message out every 40 minutes or so. The satellites are only on a 56 degree inclination, which basically means they only go up to the 56 degrees north. So, you need to point the device at the southern sky, for the best chance of hitting a satellite. I've always been able to get a message out, within an hour. BTW, you have no way of knowing if the message went out, you just have to assume it did. So, in an emergency, turn it on, hit the 911 button, and set it down, pointing at the southern sky. It should work eventually. Better, than sitting all night. Search and Rescue's life is much easier with one of these things, no search, just rescue. I guess, just because you sent a message, doesn't mean someone's going to come though. That's the reason to have crazy friends that are willing to.
Make sure that if your tracking yourself, you put the device somewhere pointing up, and not in the bottom of you backpack. It can only go through so much material and human bodies and trees, before the signal is to weak.