I was using rack plus panniers but I saw this nice Deuter Race Exp Air backpack at a shop and decided to go back to backpacks. Ended up removing the rack and pannier to save weight on my 55 km round trip commute.

The beauty of the Deuter Race Exp Air is that the backpack doesn't actually touch your back. The backpack is suspended by a "mesh" that lets the air through and which is what rests on your back. Initially I thought this might be a gimmick but it is not. This really puts an end to the "sweaty back" when using backpacks.

I highly recommend this backpack as it is really designed for cyclists and has built in reflectors facing back and to the sides. It even has high viz rain proof cover that you can unfurl when the heavens open up. Another feature is a mounting option for a rear facing blinky.