i have the topeak mtx trunk bag http://www.topeak.com/products/Bags/MTXTrunkBagEXP
Only need to carry a couple things, use it as a trunk. NEed to bring your lunch and a change of clothes? put your food in the trunk part and your clothes in one or both of the panniers. want to do a day tour or even a weekend tour, got you covered there. if you use it with the matching rack the bag simply slides right on and clicks into place, real easy to put on and off the bike when you get to where you are going.

I don't mind a back pack for short trips in cool weather but as soon as it starts getting hot, you sweat like crazy