Just found this thread and wanted to relay my personal experiences with SPOT. I do quite a bit of solo riding (street and back country) and had been wrestling with the Sat phone/PLB choice for some time; price being the hold back. I ran across a post about SPOT in early November and, after doing a bit of review, purchased a unit for a trip across Southern Utah (the day before I left).
Long story short, I used this device (with the extra "tracking" feature enabled) for this trip and it was almost a perfect experience. Basically, I only had 1 failed OK message and that was while I was in and out of the tunnels just east of Zion - no surprise there. Aside from that, all other OK messages went through and the Track feaute had great success keeping my motion tracked for my wife/family to monitor.
My trip started in Vegas and ran up to, and into, Zion NP. Surprisingly, I had great success being tracked in Zion despite the tall, narrow canyons. After Zion, I ended up outside Bryce Canyon NP for the night and started out again the next morning. Once I got to Boulder I made a command decision to try and catch the ferry departing from Bullfrog Marina and ran down the Burr Trail and through Capital Reef NP. Canyons, switch backs, deserts to mountain passes - no issues with tracking. The rest of the trip was not a challenge for terrain (Mexican Hat, Valley of the Gods, Natural Bridges NP, Mokee digway, Monument Valley, etc) and the device work as expected.
In my view, it's the perfect solution for my needs (and for many people, I suspect). The PLB's are great but the up front cost is steep and it's either "the barns on fire, send help!" or nothing. Sat phones work great but the costs are outside of my pain threshhold. The fact that I can let my wife/family know that I am OK, that I can request non-emergency help if needed and still have 911 assistance available at the push of a button was the feature set that sold me - and it worked great.
Before I left I set my support network up to be notified of my SPOT messages/requests. We agreed that I would submit an OK message everytime I stopped for the night, started travling in the morning or was intentionally stopped for any lengthy period of time (lunch, hiking, etc). Only my wife got the OK messages. We also agreed that if I sent a Help message that would mean a non-emergency, non-medical help request and they would contact AAA and the local LEO community to report a non-emergency request for help. My wife, father and brother-in law got these messages. Any accident, injury that imoblized me or other medial trauma would be a 911 message that that goes directly to the SPOT Operations Center and they manage emercency response to my location.
My only complaint with the product is that there isn't a "guest" login feature that would allow a "read only" look at your map tracking and messages. As it is today, the ID that has access to the maps also has full access to managing the account. For this reason, only my wife has access to the maps.
While I have read about Globestar's problems with Sat voice services I also read several opinions that indicated the data portion of their network was much more reliable. I hope that remains the case. Of course, if/when an Iridium-based solution comes along with similar features I will likely swich to get the (potentially) better coverage. I now carry this device anytime I go off the beaten path and I feel a lot more confident know that if I get into trouble I have a very good chance of getting help.
So far, highly recommended!