I've got the Lg Banjo Bros. bag too. The liner is what makes it waterproof. It's removable, so if you ever puncture the liner, you can get another one w/o replacing the whole bag.
It is lacking in the organization department, but I was willing to sacrifice for the waterproofness. (One of my softcover textbooks got wet in my regular bag.) Now that I'm taking most of my classes online, it's overkill and will become my 'beater' pack.