The Western Flyer from Tom Bihn is a small duffel/backpack conversion travel bag designed for quick trips or minimal travelers.
Other Bags to Check Out:
The Monolith Weekender from Heimplanet is one of my favorites because it is very durable and simple. Carries all your shit, and comfortably!
The Black Hole Duffel from Patagonia is called the "black hole" because it fan fit all your stuff.
The Mover Pack is an affordable duffel with simple backpack straps, solid materials and a few solid pockets for organization.
The GR2 from GoRuck will last your entire life, features a segmented main compartment, is extensible with pouches and accessories and boasts a very rugged, handsome look.
The Aer Duffel Pack is not the tool for every job, but for particular uses it just might be perfect for you.