I already have too many backpacks, and so I didn't want to dish out the money for a motorcycle specific backpack. I already had a maxpedition Vulture II backpack, which is basically a military 3 day pack with pals webbing.
46 liters, so not exactly a daily commuter. But has quick adjustable straps, sternum strap, and a convenient hideaway padded waist belt Also has compression straps. So i can carry a lot of stuff, take it off and put it on easily in full gear.
For daily type stuff (now that my tank bag is no longer with me), I use my mongo pack, also from Maxpedition.
Sure, I'd LOVE to simply have a un-stealable but affordable tail bag that i can walk away from during a stop, but we all know the only feasible way is hard luggage, which i can't invest in right now.