Certainly seems to be a lot of sweaty commuters in this thread. I live in South Florida and use a backpack almost exclusively, even in ~90 degree heat with 70%+ humidity in the summer. I'm going to sweat, regardless of whether I have a backpack on or not, just by walking out the door. I've found that most quality backpacks have design elements to the back and harness to maximize airflow as much as possible. Economy packs or something designed to be a student style book/laptop bag generally don't have much in the way of harness or backing features for comfort or airflow. Hip belts can relieve the weight on those with back issues (I have 2 herniated discs myself, but the backpack doesn't really aggravate the condition) by placing more of the weight carrying load on your hips. While a backpack is certainly going to trap more heat and moisture, regardless of the design, than going with panniers or a crate/basket, the convenience factors of a backpack are overwhelming for my purposes. So, a well thought out and designed backpack can mitigate at least some of the discomfort factors many people seem to have. Of course, it's all a matter of choice and what best suits a rider's particular carrying needs and circumstances.