I have beans in my bags, one for my neck and one for my feet. I think they are navy beans. I have been using the same ones for at least two years. At first they STUNK for a bit after they were heated, but that smell doesn't happen anymore.
The bag for my feet is a piece from an old blanket. It has gotten too hot at some point and has lots of burn marks and even a hole in it. It still works. I put it in the microwave for 99 seconds now and that seems to be just right for MY microwave. I still feel a little moisture every time I heat them.