I have had it switched on on my watch for over 5 years at this point. I record activities pretty much everyday which over the years has been a combination of both running and cycling. In that time I have probably has around 5 false positives, 3 of which I have notices and cancelled before it has sent messages to my family.
I have had a couple of actual crashes in that time and it picked both of them up fine and sent messages as it should do, they were both on the bike. It has never detected any falls I have had whilst running however.
My false positives have never been in the same place, so I keep it on as the couple of times I have had crashes it has worked as intended and although I was fine and able to walk away from them if I hadn't I believe it could be beneficial still.