I had a Spot 2 with me on my 5 weeks (hiking and packraft) in West Greenland this summer - short version: only about 60% of my messages arrived.
I had to exchange my new device immediately after it came to me because of these technical problems and after around 6 months (!) I received an exchange Spot2 - obviously not new, but a used one.
I sent a mix of OK and custom messages (set up camp, etc.) every evening, interesting points - I am not sure yet if there is a difference between the two, but could be.
Even in a perfect clear sky, sunny, no canyons, no trees, waiting for at least 20 minutes until the light is of - only 60% of the messages came through.
Worst were 4 or 5 days in a row without messages! I didn't use the tracking function - it is obviously more redundancy, sending only 1 or 2 messages per day seems not enough for reliabilty.
Maybe it is because Greenland is on the edge of the served area?
The label and colour on the two buttons for Help and SOS is already disappearing.
I still think it is a good thing and I would recommend it too, but knowing and accepting that it is not perfectly reliable should be part of the deal.