While not the most stylish, duffle bags definitely have their place while traveling. In particular, a duffle bag is ideal for holding awkward shaped items that tend not to fit or simply take too much space in a traditional piece of rolling luggage.
However, be sure that what you plan on carrying is lightweight. A large, heavy duffle bag is no fun to haul around. Because of this, for general travel beyond the local gym, a rolling duffle bag might be more practical.
All the duffle bags below are available through Zappos.