The SPOT Tracker does look rather interesting, although I'd be a bit worried about triggering the 911 button inadvertently, since it looks like once it's turned on (which I guess it would have to be if you're using the SPOTcasting feature) all you have to do it hold down the button for a couple of seconds. I would have thought that this could easily happen if the unit was in a pocket or bag.
I think I'd prefer it if you had a covered button, or it required some more complex series of operations to trigger it, especially since the only thing that appears to indicate that it has been triggered is a flashing LED.