Friends have used them on remote rafting trips. The large engineering firm I work for requires their use at remote sites for safety reasons.
They work. But they cost much more per unit, per minute, and are bulkier than cell phones. Prearranged messages help make for clearer communication. SPOT lets you say "OK" much more cheaply and with a far smaller unit, except when it doesn't. Some family members can grasp the concept that good news is good news, bad news is bad news but NO NEWS ISN'T BAD NEWS. Many family members can't grasp that and then SPOT creates more problems than it solves. If a sat phone doesn't connect, the sender knows it. But with SPOT, you're flying blind.
I used SPOT for a few years then went to a PLB because it's more reliable at high latitudes and there's no annual fee. I can't send messages to family, just call out SAR, but that suffices for me. And knowing I can do that satisfies the homefront.