Don't buy the SPOT system if you expect it to send tracking details while in flight. It does not work in flight, only when you are almost stationary on the ground. I purchased a unit from Aircraft Spruce and flew from Brisbane to Perth. Although set in tracking mode, it did not send position details. It did however work really well when I pushed the OK button after landing, sending an SMS to a nominated cell phone and showing the position on Google earth. I continue to use SPOT configuring the OK message to read 'I have landed at the planned airfield' and the HELP message to say that 'I have had to divert to an alternative airfield'.
The SPOT system does not do the same job as Spider tracks which is a far superior system if you require high quality tracking. I will continue to use SPOT as the SMS/spot fix after landing is great and reassuring for my spouse when I am in remote parts of outback Australia.