When I read what you wanted to use the panniers for I thought immediately of the Ortliebs too - I see lots of people around here using the roller style ones their regular commutes - some using pannier racks, some using the backpack converter. They are super tough and super waterproof, if kind of pricey.

I really like my panniers, but the kind I have are a little small for grocery shopping, unless you're only picking up a few things - one thing I have learned you want to look for when selecting panniers is the attachment at the rack - some of the panniers we have just have rubberized hooks at the rack top - they don't stay put very well. My favorite set has a big clip that very positively engages with the rack and locks it on - stays on much better, even if the little hook at the bottom of the bag that is supposed to control sway pops off.