I carry clothes+laptop in the Arkel Commuter (similar) which gets carried in to the office and goes with me when I stop at the store. But I also have a cheaper Topeak bag which stays on the bike and carries my rain jacket, random cables and straps, etc. and my U-lock has a place it sits on the rack. I've started leaving a couple of pairs of shoes at work just to avoid packing them back and forth.

I can cram all that stuff in the Commuter but it's just too much of a hassle to do it every darn day.