I use a large saddlebag to carry my gear back and forth to work. I teach 5th grade so I had to make sure I got a bag that was big enough to carry my largest textbook. I don't think I've ever filled the thing to capacity. I went the saddlebag route because I didn't have to worry about balancing the load. The material is pretty water resistant. I've only been poured on once (so far) and everything stayed dry. Follow the link and you can see my bike with its bag from a couple different angles.
http://www.tandemhearts.com/bike/ramb/index.html
Arkel also makes really nice bags. Very sturdy. We have one of their handlebar bags for our tandem. Everything in it stayed dry when we were caught in a tropical storm in Hawaii.
Happy shopping!
Veronica