SPOT is an excellent investment for anyone going off the beaten track. It works well. Battery life is amazing. One thing I really like about the SPOT is the ten minute position updates.
A worst case scenario is crashing your bike in such a way that the SPOT is destroyed and you are crippled and unconscious. If your friends or family are following your SPOT updates and updates suddenly stop, then at least your watchers will have a good idea of where to search for you. This is a huge advantage over a PLB device.
I strongly suggest SPOT users check out the excellent free spotwalla.com
Spotwalla provides a superior map display of your SPOT tracks, and (unlike the official SPOT site) saves them forever, giving you a nice souvenir of your ride.