The day I tried strapping a backpack to the rack. I had been fussing with different racks and panniers and trying to dial in an easy, lighter system. When I got about half a mile to my destination, it nearly fell off, so I decided to just throw it on my back for the remaining bit and worry about it later. It only took that short half mile jaunt to realize racks were SO not for me.

Alternatively, I did buy a medium sized saddle bag to keep on my nice bike so I can stash a jacket, tools, u-lock, etc. for longish rides with friends where I need more than I want to stuff in my jersey pockets. That works great as well.

EDIT: I do think I'd like to get one of those larger saddle bags you see the bikepacking folks using. I'd definitely like to try it out on a lightweight tour.