I was looking for eco-friendly canvas bags due to concerns with lead and other chemicals potentially found in "environmentally friendly" reusable bags that you can buy in the stores.  I bought several of these bags (natural) about a year ago, and they have held up well.  I am ordering more, now that stores are charging fees for paper bags.  They do not have a flat bottom and seem a bit smaller than the usual reusable bag, but they are sturdy, wash well, and can still hold quite a bit.

Just a short update: It's now several years later, and although I have replaced a couple due to wear and tear, I still love them, and use them all the time.

Update 3/26/16: I have not had any of these break. Still use them ALL the time - they are wonderful! I currently have half a dozen and wash them once a month or so.