We have 2 x spot messenger units that we use to suplement our very expensive irridium based tracking units.
by all accounts the spot appears to be just as reliable and at a fraction of the cost. the spot uses the globalstar satelites so does not have the same coverage area as the irridium network but unless you are going to be flying in antartica or over the central pacific ocean this should not be a problem.
We primarly use the spot to track the progress of our aircraft.
the company website is
www.findmespot.com