Just to add another option...
Avenir Metro (Amazon link). They aren't waterproof, and the unneeded bungee cord that attaches to the side of the rack will break, but the bags themselves have held up well with regular commuting (clothes and lunch) and I just wrap things inside of a plastic bag on rainy days.
I load them up with a change of clothes, packed lunch, u-lock, keys/wallet/phone, and a spare tube. They can also fit shoes, but I just keep a pair at work. With the shoes, they're packed to the limit.