I use the midsize one for errands around town. I have found that it doesn't like to stay on my back but keeps swinging wildly off to the side while big ol' Bubba's bag stays firmly in place on his back. I'm guessing it is due to the difference in our size. I now use a BOB trailer for errands most of the time, but a trailer seems a little overkill for commuting not to mention storage. I would rather use racks and bags or a daypack.