I considered one of the SPOT devices since they seem to get the most attention but after a couple days of research chose the ACR ResQLink PLB. Although a little more expensive up-front, there is no annual fee for monitoring so it’s much less expensive in the long run. The main decider though was that the ResQLink is a real Personal Locator Beacon, unlike the SPOT devices, and I have more confidence in it getting someone rescued.
The ResQLink is the same technology that has been used for years in aviation and maritime applications and connects to the the same proven satellite network and infrastructure that helps find crashed planes and lost boats. Its sole purpose is to summon emergency help – it doesn’t have the “keep track of where I’ve been” feature that newer SPOTs have, nor does it have the ability to send text messages (though there is a service that tries to piggy-back that feature on top of the self-test mode, with mixed results and an annual fee).
The device is registered to me and the registration database has contact information for friends & family. I can also enter my itinerary and any other information that could be useful to search and rescue crews – like the the channel and code I’m using if I’m carrying my FRS/GMRS radio.
My understanding of the basic operation is that once I activate the device it sends a 406 MHz distress signal at 5 watts every 50 seconds that is (hopefully) received by a satellite. This signal contains my personal identifier and also a location coordinate if the GPS receiver was able to obtain it. If a GPS fix couldn’t be obtained then the satellites will probably triangulate my position eventually anyhow.
The ResQLink and most/all other PLBs also transmit a continuous signal at 121.5 MHz at 50 milliwatts that search and rescue crews can use as a homing beacon once they are in the right vicinity even if they didn’t get coordinates with the main 406 MHz signal.
Finally, the ResQLink will activate a white LED strobe. All 3 signals continue until the device is turned-off or the battery dies (about 30 hours?). The battery cannot be replaced nor recharged by me – it has to go back to the factory for replacement after 5-6 years (or after being activated). I’m hoping I never get to report on how well it works!
Sep 18 2013
Sep 16 2019 update: I’ve completed my first battery replacement cycle. The cost was $155 – $80 for parts and $75 for labor. I’m guessing the turnaround time is normally 3-4 weeks. In my case the process was interrupted by me being out of town for about 2 weeks during which I didn’t want the unit shipped back to me. A little internet searching reveals what appear to be less expensive ways to replace the batteries but having it done by ACR results in the unit undergoing their bench tests and assurance that it still functions properly.
The product was purchased with my own money. I have no material connection with any companies, products or services mentioned in this post.
The Compulsive Hiker 
Purchased the ACR PLB 4-5 years ago as I often backcountry hunt several miles from the road in areas that have no cell phone reception. I purchased this device for the same reasons you listed, looking for a device thats sole purpose was to save me in a situation where I was unable to save myself and where I had no other modes of communication for whatever reason. I did not want a toy nor something that would help me update my Facebook page. I needed something that was bombproof and would summon the cavalry without question and with ease. I have never looked back on this as a poor purchase and believe this is prudent insurance if you are destined for remote places.
Thanks for sharing your view. 4+ years later, I have no regrets though it has still never been activated.