I commute on a full-suss MTB, so panniers aren't even a choice. Don't think I'd choose them, anyway, I prefer the weight on ME. I can use it for body English, instead of having to work against it. Large loads get bags hanging off the handlebar, or the trailer.

Have a MONSTER-good backpack; carried my tools/parts, rainsuit, work supplies, a 100-oz bottle of laundry soap, and work shoes -- didn't feel the weight til I got off the bike.

Oh -- the commute, depending on conditions, can be anywhere from 2 to 9 miles one-way; the above-described ride was an old job location, 9 miles was the SHORT route.