Spin through the Navigation Thread, I talked about them more extensively there.
Short(Er)Version- They can will save your life, especially if you do mostly solo, off-trail travel as I do. In the cracks and crevices of the Desert I could be 2 miles off hard ball at Lake Mead and it could take a few days to find me.
The Unit is about $400-500.
Garmin is still the Best IMHO, they bought out Delorme.
I have the $15 a month plan.
once I turn on tracking and send my wife the checking in text/message, when she opens that text it will give her a Sat map picture of my exact location w/in 50m I believe.
The Garmin allows you to communicate with SAR once activated, you can advise them on what they are coming into vs just a distress call like other units. Ie: CPR in progress, fall requiring rope/helicopter extraction, etc.
When Satellite phones became affordable I was going to get one until I found out your $1.50 a min, minutes you buy expire! That might have changed? My Delorme is 5-6 yrs old and still works like a champ. I never leave home w/o now.