Does anyone have an Ortlieb bag with the QL3 attachment system? I'm curious to know if you can still attach other panniers to your rack without taking off that mounted bar.

After about 10,000 miles my old Axiom Lasalles aren't looking so hot. I'm looking to upgrade to something that looks a little sharper and would be easier to carry, because I generally lock up my bike for the day and then walk a LOT. I was looking at the Ortlieb Downtown panniers and I'm wondering how well that attachment system will fit into my life; I own other panniers already that I use for errands and touring that I'll still be using on a regular basis. I'm also looking hard at the Timbuk2 Shift bag. If anyone has any other suggestions on laptop bags that will attach to my rack, I'm all ears.