Ive looked at the Bug before and dont think its a very good option. First, its a lot of money for a non-waterproof bag. Second, I would be concerned about having to put the back side of a pannier against my pack after a water, muddy, or slushy ride. Currently, I have baskets on the bike which allow me to put my Chrome messenger bag inside of (which is waterproof and can also be worn on the bike should I need extra storage in the baskets).

The second one looks interesting but has all of the same faults.