OP, I use one called Medical Guardian for my husband who is disabled. It is one of the more expensive ones on the market. I think it is about $540 for the year.
And.. so far we have never had to use it for real. But I think the best time to rely on an alert system like this is precisely when you almost never are using it.
As PP says, if you start having to use it more than once or twice, that means it is time for a different living situation. It really is providing peace of mind for someone who would be helping the person if they were home with them, but can't always be around.
Anyhow - what I live about Medical Guardian is that when we test the alert system (I test monthly) they always answer immediately. Once we tested the alert button but then for some reason I wasn't able to make it in time to tell them it was just a test. When they heard no response, they immediately did the nest thing and called my cell phone. I was able to tell them via cell phone that it was just a test. If they didn't reach my cell phone, the next step was to call 911. I feel a lot more comfortable with my husband's ability to get help by having them call 911 than by him trying to do it himself.