I started with a Vaude Sienna backpack. It has a clever frame design with a web that holds most of the area away from the riders back...a breathing space that prevents trapping body heat and sweat. It also has waist and sternum straps that keep the pack from shifting no matter how you maneuver.

As good as that was (I think it's the best cycling pack I've seen), I found hauling heavy loads tiring when riding with dropbars (strictly roadie) or I had a hulking pack with just a few items in it.

I went to the Topeak trunk bag with one of their tubular racks that has a Quick Track mount for the bag. You can read about it on their website, but the short version is the bag can be mounted or removed in two seconds and is rock solid on the frame. The bag can normally carry the equivalent of 8 beverage cans, but also has a zippered gusset that nearly doubles the capacity. On my commute I can easily pack food containers for breakfast and lunch, some clothes and other necessities. The zippers are very well made, too, an important point for something that gets heavy use (and frequent overstuffing) for years.

The Sienna still gets used if I have to carry something unusually bulky (a laptop, sports gear). I'll get around to panniers when the price is right (Arkel T-42's), but for now I have a great combination.