If I had a long commute, I'd be rocking the rack and panniers. Likewise for large, heavy loads. But, since my commute is just 3.5miles each way a backpack with a laptop sleeve and good padding fits the bill. The most I normally carry to work and back is a laptop, power supply, and lunch. On occasion there might be a growler of some delicious Sierra Nevada or homebrew.

I use a North Face Big Shot which has some nice mesh back padding that allows some airflow. It still causes a sweaty back on the super hot days but not nearly as bad as my old backpack. One thing I really like about a backpack is having my hands free. When I get off my bike and lock it up, I can just walk into a store or wherever without carrying a big bag around in my hand.