My commute is 12 miles one-way and I'm currently alternating between a Mission Workshop Shed and an Alchemy Goods bag that's no longer made. The Shed is pretty huge, but very comfortable. I find that both messenger bags are more comfortable than my Chrome Barrage backpack.
Last year I picked up a Timbuk2 Especial Claro and it didn't last one day at Interbike. In addition to being extremely uncomfortable, one of the seams blew out and the strap started falling apart. Quite disappointing as my old Timbuk2 bags (circa mid-90s) were pretty bomber.