I love them. I use them ALL the time - for EVERYTHING!! (library, grocery shopping, piano lessons....etc!)
But, I was shocked - horrified, really - when I heard that they may not be good to use - say WHAT??!!
Here's the scoop: make sure to wash them - frequently. This is the problem: if you don't wash them then you are (possibly) giving bacteria the chance to grow and thus contaminating other foods. So, the simple solution is to wash your bags!