I have three bikes and three racks:
A Topeak explorer on the Kona.
A Racktime Addit on the Univega.
A Blackburn knockoff that came with the Univega on the Fuji.

If I have a lot of stuff, I use my panniers. If I don't have a lot of stuff but have an appointment (sweaty back = bad), I bungee a backpack to the top of the rack. If I'm going into work with not so much stuff (16 miles one way), I use my backpack on my back because it won't affect the handling on the rear wheel.

I like the look of that Ortlieb bag, though. I might have to get myself one of those.