There are lots of good options for trunk bags. It all depends on whether you want to use a rear rack and don't mind the extra weight. With a rack, you have the option of getting panniers later if you need more carrying capacity. Personally I like the Carradice bags and have been using a Barley for commuting the past 5 years. It holds everything I need 99% of the time. What's nice about the Carradice bags are that they center the weight near your saddle so the weight doesn't affect handling. They also come in a range of sizes. If you go the Carradice route, however, I would recommend getting one of their Bagman racks (which attach to the saddle rails), which support the bag and keeps it from hitting your legs.