I got the skidoo one that fits on the tunnel, i wear it as a backpack when i ride locally, which is flat land. When i go to the hills i wear an abs pack and strap the skidoo pack onto the tunnel .
I ride with the HMK pack, I have shovel and probe in it, along with many other things. I find them very handy, I don't like a lot of weight in it, so I also have a bag on my tunnel for my lunch, drinks, extra clothes, gloves, a few tools. I just keep the life saveing stuff in my back pack.