I think PLBs (Personal Locator Beacons) are an excellent idea.
They are not cheap, so they are less likely to be carried by a casual or inexperienced walker. So those people who have them probably know what they are doing to a certain extent, so they are less likely to trigger one in anything less than a life threatening situation.
PLBs and Spot Messengers are now being carried in large numbers in the UK. There have been several activations in the last 12 months, all of which were genuine (apart from one that heard about. It was a stolen device that was set off by the thief in an urban area, who I am guessing got a big surprise when the Police turned up!). There is one very big disadvantage to most of these devices. The rescue teams have no idea what issue you have or how many people are involved. They just know someone (or a group) is in distress. The location data sent to the rescue teams is usually very accurate on modern beacons as it is GPS derived.
Some of the devices allow a text message to be sent, so they could convey details of the issue.
I carry a McMurdo FastFind. It’s small, light and the battery has a sealed storage life of 5 years. Other brands are available!
This web article discusses the various satellite beacon technologies.
The products are not what to look at, consider the technologies and what you want.
I went for a PLB as I wanted an emergency only device, which gave the highest chance of being picked up and no ongoing contract. If you want non-emergency tracking or messaging this is not for you.
EDIT: Please remember that it is much better to get a message out to the emergency services by phone, either voice or SMS. In fringe coverage areas SMS will often get through when a voice call will not.
If you wish to send an SMS to 999 you need to register your phone first. Details below. You do not need to be deaf to use this service even though it is supported by the RNID.