It's been ok for me for the last 4 years. But I only carry the minimum in it. Just work clothes for the day, a few tools (Park multi allen wrench, 15mm wrench, spoke wrench) and flat fixing tools (tube, mini-pump, 3 tire levers). I get away with leaving my lock on the bike rack at work. Also get away with not carring a laptop since I have one at home and a desktop at work. Hope that explains it. I only ever have to pack the backpack tightly in the winter. That's because I need to wear more layers of cycling clothing then. But I could get away with a little less than 23L if I left a plastic bag in the backpack. Then I could just use the plastic bag for the 5-10 min that I need to carry the clothing from the men's room to the coat rack on the other side of my floor to hang them up. Oh, for shoes I leave a pair at work. I put my cycling shoes in a desk drawer during the day. Finally, for the front and rear lights, I leave the charger at work. Then just leave them plugged in during the day at my desk.

Edit: For rainy days, I'll take two plastic bags from home. One will be used to hold my work clothes in the backpack. The other will be used to cover the seat. The plastic bag seat cover may seem a bit ghetto, but its effective in keeping the seat dry. I learned that from the bike delivery guys here in NYC.