I've been using the same old Timberland backpack as my disc golf bag since 2002 when the main zipper broke and I could no longer use it for carrying books/papers... but the discs fit in their great, it's well made, comfortable, and very light. For a tournament, I usually have 14 discs, plus water bottle, a few snacks, an emergency Mt. Dew, and a towel or two in there...
The only problem is it sometimes tries to roll away when I set it down on hills because it kind of molds around the round shape of the discs depending on how things are arranged -- I've had to chase it once or twice. But I've promised myself I'm not buying a real disc golf bag... I have to get good enough and win one before I deserve to carry one.