I have one, have worn it most of the time when alone at home for the past 2 years or so. I feel much more secure knowing that if I should fall and be unable to get up, or something else awful like that happens, at least I might have a bit more of a chance of summoning help.
My unit says Greatcall on it. Not sure if that's the name of the company. It costs about $28 a month. It seems to work well; I've fortunately never needed it, but have tested it a few times, and a voice always answers immediately asking if I need help. They don't mind if it's a test or a false alarm, in fact, they encourage you to test it regularly.
If you do need help, they call 911 for you. I guess if you happen to be next to a phone, or have your cell phone in your hand, you can just as easily call 911 yourself. This is just additional help for those times when you can't get to a phone.
In the shower, I put it in a plastic bag to keep it dry, as I don't like wearing a wet cord around my neck. But I do want the thing in the shower, as that may be one of the main places where people fall.