Read the conclusion, this is a pretty good review of spot.

http://www.trailspace.com/gear/guide/help-find-me-spot-gear-test.html#conclusions

In practice, these compromises result in skipped updates in Track messages and times when OK and Help messages do not get through. In some cases, I observed gaps of more than 30 minutes, up to over an hour in Track message sequences. Users of the present generation can greatly improve the message transfer success rate by making sure that the SPOT is held horizontal with the SPOT logo facing directly vertically.One unsuccessful rescue generated a lot of overly sensational press coverage. There is risk in everything in life, and no electronic widget is going to completely remove that risk or guarantee life and limb.