If you sit upright on your bike, a messenger bag is fine if it can hold everything you want. When I switched from my cruiser to a road bike, the messenger bag kept shifting while riding, and it only took a minute or too before the bag slid from my back and was hovering on my side near the handlebars (in a horrible position). I've since switched to a backpack and couldn't be happier. Of course, I haven't tried strapping a bag to a back rack, as I don't have a rack.