I have, and used to use, an medium duffel.  It's great, but I like this one better.

While small, this one still holds my travel necessities for a week.  I pack a pair of shoes, travel dishes (like you get for camping), a pair of pants, blouses, t-shirts, nightgown, undergarments, toiletries, clock, etc.  I use this for vacations since I stay in motels with a fridge and microwave.

There is room for a book or two after I am done packing.

However, I should mention that I tend to drive on my vacations, so there is room in the car for the other ten books I take with me.

If you are using this for a gym bag, I think it would be great.  If you are using it for a travel bag, yes...just don't take an enclopedia with you unless you are driving and have room in the trunk of your car.