Being a NYC resident, I make frequent but small runs to the grocery store. I never buy more than I can carry since I'm on foot. Those trips to the store are rarely planned though, so I often don't have a reusable bag with me.

Now I've started keeping an Environsax bag in my various coats and back packs. (This came with 5.) They're so darn small that I doubted they could adequately hold my groceries around without breaking. I was wrong. These things are sturdy bags and quite large too. They look much better than the typical burlap grocery bag too. Highly recommended.