I've been using my SPOT for two years. It's a little sketchy in narrow canyons and under tree cover. On extended trips I check in twice a day, morning and evening. Dayhiking I check in when I reach the trailhead, this lets my wife know where I'm parked and again at lunch.
It may not be perfect, but I like it and my wife appreciates having a little more info on my location.

Don