Ive been lucky enough to use one, some years ago, but their only disadvantage, is that they cost an absolute fortune to buy.
A really good one, costs in excess of a £1000, but as with all technology, prices are coming down, but even a no thrills version costs many hundreds, so unless its vital you require a mobile signal, keep using your mobile phone.
They work almost everywhere, using the GPs global positioning satellites in space, but unlike our Garmin GPs units, the satellite GPs will allow data and messages to be transferred around the planet.
I am sure many of us have watched Everest summits use them to phone home whilst on top of the world.
I recon the technology, will remain expensive for quite some time, as since America owns virtually all the satellites, they want to ensure those who want to use them, pay dearly for the pleasure.
It was one of my friends in South Wales who owned a Motorola model, and it was straightforward to use, but at the time, very expensive to operate, around £1.50 a minute.
Its a good thing the technology exists, but as they have been around for some time, its unlikely the technology will become mainstream, like our mobile phones.