I bought a couple, but I usually forget to bring them into the store. Eventually I'll get into the habit.
My grocery store gives a nickel discount per bag if I bring my own. I am pretty sure it costs less than a nickel to buy a throwaway bag, so my guess is they actually care, or at least it's a marketing idea to get green folks to shop there more. But, I shop at an organic food store, so that might make a difference.