I'm going on a cross country tour this summer from Vermont to Oregon and got this bag so that I could try and set everything on my back rack as opposed to buying a front rack and 2 more panniers. This bag is large! I can fit a tent, a pillow, a sleeping bag, my sleeping mat, and all of my cookware plus my repair tools in here and still have plenty of room for miscellaneous items. I can even fit a ukulele and a trumpet in it on top of that! There are hooks on the sides so that you an give it a more geometrical feel so that it sits sturdy on the top of your rear rack and panniers. There are also slots in the top so that you can clip caribeaners to it or slide bungy chords through it to keep it in place.
This also doubles as a backpack! It comes with 2 very comfortable adjustable traps which can be velcroed together when you don't need them or loosened if you need to wear it.