I have a Spot and use it when I am out on my own. It can, however, be a pain to use. You have to be careful what button you push and it isn't obvious when you are in tracking mode. It only has 4 buttons and it's easy to think you're in tracking mode when you're not (have to watch the flashing lights). It also needs a pretty clear view of the sky or it will not work. To use it on a bike you would have to mount it in the open or wear it on a belt.
I am not too impressed with tracking. They could do better. I do like the fact that you can use it to call for help and let family know you're ok. If you're out on the road and cell doesn't work, Spot probably will. It works virtually everywhere in the world. Press the 911 button and they will send assistance to your GPS coordinates. It isn't perfect, but it's certainly better than nothing.