I have one. They are fantastic (in my opinion). There is a small monthly fee to maintain the service and you can send as many text messages/emails as you want of the 3 presents. You can also preset 20 others or free-form it and you get some of those free a month depending on what plan you are on. It will link up with your cell phone through blue-tooth and the tracking is done through the unit saving your cell battery (which you can place in airplane mode). The people you nominate can track your location through computer or smartphone. They say the battery in the unit is good for 100 hours. When I have gone out with it on an 8 hour patrol, tracking update set at every 10 minutes the battery has gone down to 92%. I haven’t needed the SOS feature (and I hope I don’t) but it gives peace of mind for yourself and anyone that you wish to let know. Highly recommend.