Just to agree with drewitgolf. My first encounter with the Impact Bag was when I took a few lessons with Ben Doyle. He had at least 3 of them laying around the range and we used them extensively. They were filled with old towels. I picked one up myself & used the same material.
Excellent place to put golf towels that have seen their best days.
I guess if you put something heavier in it you risk wrist injuries and the bag may crack up.
It does move substanially at times. I guess it is only a good thing to have to practice taking a new grip and stance every time.