I have one friend who sails to the Bahamas this time each year, and everyone - friends and family- can track his progress and know his whereabouts and be alerted if something happens out in the middle of the Gulf of Mexico and the Atlantic. It's a nice feature to be able to notify someone if you're in trouble or at least to let people know where you are.
Another friend of mine last year met up with a friend on the way and they rode their bikes to the rally in Sturgis. He could be tracked every day also. I kept teasing his wife that she knew where his Spot Messenger was, but she had no idea where he was.
Didn't go over too well but I thought that was pretty funny.
You have to figure out what you want to use it for. If you do serious offroad riding, in the woods or desert and you need help, they can find you real quick with the push of a button. You could probably shackle it to your kids or hide it in the car if it had a view of the sky and track them or your wife every 10 min.
It would be nice if the service could be activated and deactivated when you want to use it on a trip, instead of paying for the whole year and have it sitting in the closet most of the time.
You can keep track of your trips on Google maps, but you could do that with many standard GPS units and download it to your computer if you wanted to.
It's just any other gadget we all buy but it's a good safety item if you can use it as such.