I haven't used one, but it does sound like a potential life-saver. Currently, I rely on researching the dive shop I go out with to ensure they are reputable and won't leave a diver behind. Most of the places I've been diving, you go to areas with multiple boats and they do a good gob in checking people on and off the boat. Additionally, if you are diving with a guide, and truly using the buddy system, you are hopefully not going to get yourself into a situation where you are lost. Finally, I always carry a safety sausage and whistle to ensure I can be heard and seen if I ever get too far away from the boat and do not feel like I can swim back in. I imagine even if I had a GPS distress signal, I would still carry both of those items.