5:49 p.m. on February 16, 2012 (EST)
Ok, here is a bit of explanation about the SPOT family. I have and have used the Tracker, Messenger, and Communicator, though not the Connect (doesn't work with my Blackberry or my very basic iPad 1):
All SPOT units currently on the market and in use are one-way only. All the units have a GPS chipset in them, though the Connect and Communicator send the location of the controlling device (iPhone or Android phone for Connect, Delorme PN-60w for the Communicator). The message is relayed through the GlobalStar satellite system, using their digital service (uses less power than the GlobalStar satphone service). Coverage is basically limited to the region between 80 deg N and S latitudes, continents and near-continent ocean areas. Antarctica is not covered, nor are the southern parts of South America and Africa. SPOT recommends allowing 20 minutes of attempts to get messages through (obviously, for the 911/SOS messages, you turn on the emergency button and leave it on for the approximately 24 hours of continuous transmission battery life).
SPOT 1 (SPOT Personal Tracker) was the first version that came out. I reviewed it and PLBs here. It was fairly basic by current standards. You can send 4 types of messages with location - an "OK" message or a basic "Help" message (non-emergency) to your "team", track points that anyone having the URL for your Shared Map, or a 911/SOS message that goes to a S&R coordinating center that will notify the nearest official SAR agency to your location. The GPS chipset and antenna system are a bit weak in performance compared to the later units.
SPOT 2 (SPOT Satellite Personal Messenger) has a lot of advantages over SPOT 1, the biggest being that the performance is far better both in getting messages through and in battery life (though the battery life is plenty long in normal use). It is smaller in size than the SPOT 1. I have gotten messages through from Puerto Montt and Santiago in Chile and Easter Island. In addition to the messages that SPOT 1 allows, you can pre-program a custom "OK" message (("Ok but delayed" or "Pick me up at these coordinates", for example). SPOT 2 is significantly better at getting messages through under canopy and in canyons than SPOT 1.
The SPOT Communicator allows a lot more messages when used with a Delorme PN-60w GPS receiver. The SOS/911 message can be sent to the S&R coordination service autonomously. For use with the PN-60w, you can set up a number of pre-programmed messages (along with the basic "OK", "Help", "Track "(via the "Shared Map"), and SOS/911 messages), such as "OK, but delayed", "at the trailhead, pick me up here", "reached the summit" and so on. The Communicator also allows "type and send" free-form messages.The coordinates sent with the messages are the PN-60w location, except for the autonomous SOS/911 message, which uses the Communicator's GPS chipset. The Communicator allows setting up several "teams" that you can choose among for sending the messages.
The SPOT Connect is basically like the Communicator, except that it requires an iPhone or one of a group of compatible Android phones. The iPhone or Android phone must have a GPS chipset to be compatible with the Connect (not all do).
There is an annual service fee of $100. When you activate the SPOT, there is a note that is easy to overlook that your credit card is charged automatically each anniversary date to assure continuous service. In other words, if you want to discontinue the service, you have to directly notify SPOT to shut it off. There is also a rescue insurance fee that you can sign up for.
SPOT can have fairly long outages, but usually gets the message through in the covered regions in 20 minutes. I did not get any messages through from Peru, despite being within 8 degrees of the equator, though, as mentioned above, I did get messages through in Chile and from Easter Island.
There is another device that pairs with the Delorme PN-60w, called inReach. In many ways it is similar to the SPOT Communicator, but adds 2-way texting. The inReach works through the Iridium satphone system, rather than the GlobalStar satphone system. So it covers the whole Earth. There is also a version that works with Android phones. I have found that the inReach is very fast (usually gets the outgoing message through in a minute), though the incoming response can take several minutes (and, of course, you do have to have the inReach and PN-60w turned on). Incoming messages are stored in your "Explore" folder under your inReach account online, and receipt of incoming messages is speeded up if, when you arrive at a far away destination, you send a "ping" or an "I arrived" message, just something to let the system know where you are (just like with your cell phone).