I bought three 21x36" versions to make into exercise sandbags.  I filled them with wood pellets.  One I made 45 pounds, another 60 pounds, and the third 80 pounds.  There are instructions on Youtube on how to make sandbags for exercising.  I have been using them almost a year and they're excellent.  The material is very durable and easy to grip although possibly rough for some hands.  Another option for exercise sandbags are the white Navy sea bags which some say are even more durable and may be made of a more comfortable material.