I think it's a fantastic idea. Will it work while the device is locked though, or do I have to walk around with my iPhone/iPad open all the time? The page doesn't really make that clear, and it's the most important decision factor for me since I'd want to just have it passively tracking in my pocket or backpack.
It works while the device is locked. However, to save battery life, locations will be reported every 40-50 meters when app is in background. Thus no fog will be removed if you only move a few meters.
