As others have said, these things are a cool idea, but the SPOT is (in my opinion) more or less the worst of the bunch. It has relatively weak SOS transmission power (just 0.4W) and no true two-way messaging functionality.
A few better (IMHO, anyway) options:
DeLorme inReach SE
This one has two-way messaging (useful) and a somewhat stronger SOS transmission function (1.6W). If you value communication and tracking in addition to emergency SOS functionality, this is a good, balanced option. I've seen mostly positive reviews;
this one from Expedition Portal is particularly thorough.
ACR PLB-375 ResQLink+
If you don't really care about messaging/fancy online tracking maps, and just want the device that is most likely to help you in an emergency, this personal locator beacon has a much stronger signal (5W) than the SPOT or DeLorme options, is totally waterproof, and dead simple. Also, no subscription fee.