I really like messenger bags for quick trips to the grocery store, library, or coffee shop when I’m just carrying a few small items. They’re convenient and quick, and I almost always have one hanging near the door to grab on the way out when needed.
On the other hand, for commuting, weekly shopping excursions, or any trip that involves transporting a larger quantity of heavier items, I prefer to carry the load on the bike. For smaller commuting loads I carry everything in a single rear convertible pannier such as an Arkel Bug or Ortlieb Downtown. For even larger loads, I prefer to balance the weight between a pair of rear panniers and a front porteur rack.
How about you? Do you carry your commute load in a messenger bag or backpack, or do you prefer to carry the load on the bike?