Oh man, Backpacks vs. Panniers is nearly as hot of a topic as Clipless vs. Platforms in this subforum.

Panniers work on the right bike and the right conditions such as an easy place to store your bike, low theft situation, not having to carry it up and down flights of strairs, etc. When I say right bike, I mean a bike geared towards load carrying. You can put a rack on an aggressive frame if you try hard enough, but the weight will really jack up the ride quality. Been there done that. They have their place and they are awesome when they are convenient and work.

For people who like to ride a little quicker and maybe have to jump on and off trains, carry a bike up stairs, don't want to leave bags on bikes, etc, a backpack is awesome. And sure enough, there's this crazy thing that's been around for a while called "backpacking." There's this whole following behind it going back decades. They've been carrying stuff on their backs for a long, long time and have generally refined it to perfection. So people who are into this really obscure "backpacking" thing have even made lightweight packs that, when sized and packed properly, stay securely mounted to your body (not flopping around) and distribute the weight to your hips, not your shoulders. They even have ventilation to keep your back cool. Heck, some of these "backpacking" companies even started making cycling specific packs! How crazy is that? Didn't anyone call them and tell them they were stupid and backpacks on bikes are stupid?

I tried really hard to like panniers on my bikes, but they aren't the right bikes for panniers. I have a couple of very nice backpacks that I used to commute about 15 miles roundtrip for a little over 2 years in TEXAS. I did not have a problem with getting too hot. My back was sweaty, yes, but my back would be sweaty with no backpack and 1 mile of riding in the dead of summer here.