I have one and have used it once to date. It worked as described in terms of delivery, but i can tell you that a couple of my cows were wary / suspicious of me drawing up beside them in the Gator for quite some time afterwards. It must sting like a paint ball I suppose. I missed a couple of shots until I got used to aiming it correctly.
The instructions describe a pretty elaborate desensitization / conditioning process that I did not do since my cows are pretty tame. Basically, it's a way to psychologically pair getting shot with feeding time. The gun is not loud at all so I think it's more about the sensation of impact.
The product seemed to work well for control, though I think my bigger cows need two doses (one per side) which I think will be a bit trickier to pull off.