The SPOT messenger has a very limited application. It's main function is an emergency locating beacon with a very limited ability to send a predefined message to a preselected address. It's best use is for people who are traveling in remote areas outside of regular communications.
For cycle touring, you should consider under what circumstances you would use the device. Generally, the use of the rescue function is limited to cases where you are in danger of loss of life or serious injury with no other rescue options. If you are riding along roads, this presumes that you are in a situation where you are outside of cellphone coverage and you are injured with very little chance of waving down a passing motorist yet still capable of accessing and using the device. I see this as a very low probability scenario.
The ability to send "I'm OK messages" is all very nice, but unless you are planning an off-road adventure, I think it would be better, and cheaper, to take along a prepaid calling card and use a pay phone.
I have experience with the SPOT Messenger, though I would expect it does what they claim. Like any emergency locating transmitter, it uses the US governments rescue beacon satellites. If you did activate the emergency function, you can be sure that someone will come find you - at least in the US.