This may get lengthy - but I talked to the head of IT at SeaTow. They originally tested SPOT and turned it down for several reasons. According to them it is good for letting family know your GPS location. TowBoatUS uses it for that and for the second feature - non emergency distress. In other words, you are Ok but simply need a tow. It does not work for an emergency distress - where you need Coast Guard assistance. But it gets more complicated.

If you use the non-emergency distress option, meaning you need a tow, there is no way for the receiver, in this case TowBoatUS, to know whether this is a false alarm or not. They also have no way to give you a confirmation that they received the call. So you sit, hoping the thing worked, 40 miles offshore waiting for a tow boat you hope will arrive. Apparently TowBoatUS dispatches on some false alarms from time to time and SeaTow did not want wasted dispatches.

His advice was to go with a 406-PLB until the SPOT technology gets to the point that you can get a message received confirmation back. The PLB's run about $300 right now I believe.