I must disagree with the many reviewers who have basically trashed the utility of this device. For my style of group hiking and canoeing, it suits my needs better than any other device on the market.
I have owned a SPOT Tracker since just before Christmas, and I have used it in the field several times. My only complaint about the device is that a lower percentage of messages get through than I would like. As covered in detail in the review, when SPOT says you need an unimpeded view of the sky -- THEY MEAN IT. During a three-day trek in the Virgin Falls Pocket Wilderness (De Rossett, TN), only a small fraction of the "OK" tracking messages got through (I would estimate approximately the same 16% or so that the reviewers experienced). The terrain varied from mountainous, to tree cover, to canyon – with very little clear sky. The SPOT obviously had some trouble with this type of terrain – the very type of terrain I seek when I go hiking. I carried the SPOT on a lanyard around my neck; perhaps affixing it to my shoulder strap (as recommended by SPOT) would have improved the percentage a bit. I will say that EVERY TIME I have sent an OK message with the SPOT face up from a very open place (no trees, cliff walls, etc.) -- the message has gone through.
Whether a backpacker (or other adventurer) finds this device useful or not depends on his style. I, myself, do not embark upon any solo adventures; I always have two to four friends or family members with me. If I did trek solo through the bush, I would carry a satellite phone or a PLB.
The SPOT works for my group trips because I believe it extremely unlikely that all of us would be incapacitated simultaneously. Therefore, should someone in the party become too injured for us to remove, an uninjured hiker can simply take the SPOT to a very open clearing, place the SPOT face-up, activate it, and let it do its thing. I always use the "Tracking" feature while I am moving (hiking, canoeing). Again, while imperfect, it DOES get through enough to give a SAR team a good idea of where you have been.
For my style of group hiking and canoeing, I would much rather have the SPOT than a PLB. The additional one-way communication features (Tracking, OK message, Send Help message, as well as 911 message) work as advertised and, to me, are well worth the yearly fee. My wife LOVES being able to track my progress on the internet (I even leave the tracking feature on when I'm driving to and from the trail). Her peace of mind alone is worth the price of the SPOT and the yearly fee. The different "levels" of help one can request are another welcome addition.
As for the reviewers who have complained about the GLOBALSTAR telephone communications -- this is a known issue -- and affects DUPLEX (two-way) communication only, not the SPOT, which uses SIMPLEX (one-way) communication. I suspect the unreliability of the duplex communications is the reason the SPOT has chosen simplex communication – and does not have some of the handy features we SPOT owners would like (confirmation of transmission, text messaging, etc.). Perhaps when the new GLOBALSTAR constellation is up, a new version of the SPOT will have some of these nifty features.
Until that time, I must say that the SPOT – even with its limitations – suits my style of adventuring far more than a PLB or any other device currently on the market. Although I would love to have one of the highly reliable satellite phones, I simply could not justify the cost, given the limited number of days per year that I can go hiking. The SPOT fits the bill for me.