This is a post I made in another thread in Gadgets and Gear a while back:
I operate four SPOT units with my survey crews and have been testing the unit extensively for the past year (Atlantic Canada, BC, and West Africa so far). I have enough confidence in the unit now that it has become our primary "sched" device for remote crews checking in (while Iridium SAT phone is still primary means of emergency communications).
Let's first distinguish between SPOT I and SPOT II receivers. The original SPOT I is larger than the latest incarnation, the SPOT II, the SPOT I uses two AA Li batteries while the SPOT II uses three AAA Li batteries. The SPOT II also has an extra button for a user-customizable message, but this message must be defined prior to use on the SPOT website. Some SPOT II units (including my personal one) were recently recalled due to power/LED issues. The SPOT I can last up to a week with two batteries when sending three messages a day and tracking enabled for 10-12h per day. The SPOT II unit goes through batteries much quicker, I would be lucky to get 1.5-2 days of use from it on one set of batteries. As the SPOT II uses one extra battery, and the AA and AAA lithium batteries are the same price, and I would estimate it lasts 1/5 less time on three batteries than the SPOT I on two batteries, this puts the cost of running a SPOT II at roughly 7.5x higher in batteries alone (and lithium AA/AAA batteries are not cheap!), something to keep in mind. The HELP/SOS button placement on the SPOT I is not good, they are recessed in the housing, but I still worry about an accidental button press while carrying turned on in the top of a pack, so we tape over these buttons with electrical tape to protect them, the SPOT II has proper covers on these buttons which is a marked improvement. The LEDs on the SPOT I are a bit confusing, and unless you are watching it every second, it's hard to tell if the message was sent, and it's impossible to distinguish between tracking mode and OK message being sent by looking at the LEDs, this issue has been remedied in the SPOT II with a separate tracking button/LED. The SPOT II is certainly a much better overall package, but they really should have stuck with two AA batteries, this is its biggest shortcoming.
In my experience, when used correctly, SPOT works 99% of the time. To use correctly, the message must be sent while the unit is in the open, free of overhead obstructions. Our standard procedure while in the field is an OK message every three hours, with the tracking mode enabled the rest of the time.
For the price, it's hard to come up with reasons not to carry one. I use mine at work, and also at play, I did a 20km solo day hike last Friday in a fairly remote part of Newfoundland and my wife was able to watch me every step of the way (I guess that's a good thing!)
In researching the SPOT in early 2009, I was unable to find any independent scientific reviews of the system. I sent an email to the Canadian National Search and Rescue Secretariat requesting they review it, not sure if it was my doing or not, but they have a very good review of it in the latest edition of SARSCENE magazine:
http://www.nss.gc.ca/site/ss/magazine/vol18_3/406/spotchecked_e.asp
Full pdf version of magazine here, see page 15:
http://www.nss.gc.ca/site/ss/magazine/vol18_3/Vol18-3_E.pdf
edited to add:
since posting this, I have received my SPOT II back from product recall. The power issue seems to be solved and it lasts considerably longer than before, I haven't tested it extensively but it seems to be close to, or exceeds the SPOT I now.