Ortlieb Backrollers for me. I normally commute with only one, but add the second one if I know I'm stopping for groceries. On the weekends I load up with a week’s worth of vegetable in a single Backroller. Last time I fit the following with room to spare:
1 Cantaloupe
5 Grapefruits
2 onions
3 tomatoes
3 large squash
3 large zucchini
I have a Tubus Logo rack on my everyday commuter (road bike: 2005 Allez), very solid & expensive but I needed the extra clearance it provides to avoid heel strikes. I like the panniers so much on that bike that I got an inexpensive ($20) Planet Bike Eco Rack for the mountain bike I ride on the weekends with my wife. Still waiting on the second pair of Backrollers that I ordered with the Best Buy sale.
I highly recommend the Ortlieb Backrollers, great for groceries but also great bags for commuting.