My wife and I have been using the Spot 2 for over a year now, since reading the very positive review here. We don't use the tracking feature, just use it to send OK messages a couple times a day so her mother doesn't panic and send out the cavalry (we got the Spot after she almost called out SAR when we were on an overnight, because the temperature had gotten down to 20F and she was convinced we had frozen to death).
We have had 100% success in sending OK messages, EXCEPT for the trip a year ago when it sent the OK from the starting trailhead, the OK 4 days later at the ending trailhead, and NOTHING in between. We were in the White Mountains of NH and I was sending OKs every evening when we made camp and the next morning before we left. By Day 4 her Mom and my parents were frightened enough to call the State Police, who apparently didn't understand what a Spot is because they just said nobody's phone works in the Whites; don't worry about it.
I had been leaving the unit undisturbed, face up, for the 6 minutes Spot claims it needs to send the message, then turning it off to conserve battery life. This had worked fine in our previous trips, but not this one. Two changes I've made since then: I leave the unit running for at least 15 minutes, and I send a mid-day OK, usually at an overlook with a much better view of the sky than we often get in camp. We haven't had a failure since then, but I just don't trust it. We are seriously considering the new Inreach for Smartphone when our Spot subscription runs out (in order to get delivery confirmation and have two-way messaging in an emergency) but it's a lot more weight, volume, and cost.