I've got this bag from AFE. It was a present when I got my PPL.
AFE Soft Flight Case : AFE - Airplan Flight Equipment
What I like about a specific pilot bag in particular is the headset pockets. Other than that, I agree with others, any bag will do.
This bag in particular is just fine. The main compartment is big enough to store A4 binders, the large zip pocket is useful for all those loose items that you accumulate (outdoor GPS plus cables, pack of chewing gum, small flashlight, spare pen, flight timer, you name it), two headset bags on the sides, and the two open top pockets are used for stuff that I want to grab easily, such as a hi-viz jacket (mandatory at my airfield).