I discovered the Spot a few weeks ago. I'm getting one for the kids to keep in the car. Finally an affordable sat safety device. If they lie to me about where they are I can make them ping the sattelite. heh heh heh
I tested it in the car, in the city and in a few other places (parks, stores etc). The bottom line is - it needs just as much clear sky as GPS and more. So, if it is in the car - it has a high chance of not working or taking a very long time to send a message. In my car it worked reasonably well as long as it was on the dashboard right under the windshield, looking up. If it is moved even a bit under the metal roof of the car - it won't be able to send any messages. It also does not work very well under trees - no messages got through at all when I was in a park with medium density tree cover.
So, personally, I would use it on a boat (and I plan to keep it on board and operational at all times) but on land it's usefullness is limited, in my opinion.