I use a single Ortlieb pannier for commuting. With a coffee thermos, lunch, change of clothes and frequently extra rain gear it is more than half full. Often I will stop at the library, farmer's market or grocery store on the way home and fill it the rest of the way. I can also bungee any excess to the top of the rack.

The bike does ride differently with one very full pannier but it's easy to handle. Sometimes I wonder if this imbalance is putting a strain on the rear wheel and would be interested in what others think about this.