I tried a seatpost mounted rack and rack-top bag and didn't like the way it made my bike handle. The weight sits really high, so it's hard to swing you leg over the bag and it makes the bike top heavy. Like Schwinnrider recommended, a Carradice bag will hold just as much (or more) gear, cost no more and have little effect on handling. I've used a Carradice Barley (their smallest bag) for commuting the past 5 years and it holds all the gear I need 99% of the time. In contrast to the seatpost rack and bag system, the Carradice has little effect on handling because the weight is lower and more centered. I would recommend the optional Bagman seat-rail rack (or similar options) to support the bag and keep it from swaying and hitting your legs while pedaling. You can buy Carradice bags and racks direct from England with much better selection and prices than US dealers.