Flimflam

Yeah I do this with my backpack, too.

I switch between my mess bag and backpack - mostly because the mess bag has caused a torn rotator cuff (carrying 12+ beers over 10 miles multiple times in a week/weekend was apparently too much for my puny shoulder to deal with).

I'm not a messenger, I just like the mess bag for the added capacity (size of package/stuff I carry), the waterproof material and the ability to carry *huge* stuff with the flap open without stuff likely to fall out. My backpack is fairly big, but not as versatile. I'm just resting my shoulder now.

I'm also a car-free year-round commuter/transportational cyclist, I've used a backpack, a big hiking pack, a mess bag and my rack/pannier/trunk bags for rides - I prefer for long, long rides the weight being on the bike. The mess bag is nice if it's not too loaded for around town, but for general versatility and dealing with all situations, the backpack seems to rule. I wouldn't want to do much more than 50 miles with a loaded mess bag (not my bag anyway, single shoulder strap w/ stablising strap = too much weight on one shoulder for comfort). My full commute is 33 miles, I've ridden that with my mess bag and that's about the furthest I think with that.

The backsweat is a pain, but I change shirt when I get to work anyway - I'm probably going to put a rack on my Rob Roy and use my pannier or trunk bag for the winter (though the backpack = nice warmth) which is nice, but I do prefer having the bike weight-free and it all being on me (for whatever reason it seems faster/lighter that way).