I have a Chrome Kremlin messenger bag that has lasted me 10 years so far.
seriously, I've used this bag daily. I'm amazed how its held up.
over the years, I've used if from touring (how crazy is that?), commuting, groceries, and camping.
now days, I tend to fill it up with way too much junk, and I use it on my surly big dummy for the work commute, uniform, laptop, food, shoes, shower supplies, etc..
really it comes down to where you want to carry the weight. if its on your back, ultimately the pressure goes into your perineum.
I don't mind carrying a few things, but when my bag gets to around 10lbs, I choose to toss it onto the bike.