I've always had great success with the "I'm OK" function, but tracking is not ready for prime time. Tracking is most likely to succeed with activities where the unit's orientation remains fairly constant AND at least a foot away from the body, such as kayaking and biking.
Instead, I try to use the "I'm OK" function at any time I'm going to stop moving for at least 20 minutes; any time I'm crossing from one drainage to another (like going over a mountain pass); major stream crossings; and at any campsite. This replaces the tracking in that it will show a rescuer where I had been, the rough route I was following, and thus the specific area I'm most likely to be found. I always try to put the Spot in a place where it is most likely to succeed. While a 360-degree panorama isn't required, it helps. I try to put it on a rock out in an open area, which sometimes doesn't work inside a canyon.
If you only send one, it better go through. Getting into a habit of sending "I'm OK" messages several times a day greatly reduces the chances a loved one will experience anxiety because they are getting at least one each day.
I've been pleased with my unit's performance (I have the original Spot.) and am strongly considering the new version, which permits an iPhone to send a 41-character message. Not wild about taking my iPhone into the wild, but I like the ability to send signals more specific than "I'm OK," "Help," or "911."