Here is a great article about reuseable bags from Mother Nature Network. Here are two interesting quotes from the article if you cannot read all of it:
A nonwoven polypropylene bag, for example, would have to be used just 11 times to make up for the negative effects of a plastic bag used one time, according to a British Environment Agency study that compared bags. A cotton bag, however, would have to be used 131 times.Amazing.