Why this ammunition? I guess the fact that they have pics (an old ad said they had video, might be available) of a boar being taken out with a 9mm round. A 500lb boar. That's good with a rifle, but a 9mm? The reason it is able to do this is the way it fragments. The bullet fragments in a more or less controlled manner, creating potentially hundreds of wound channels and creating a large temporary cavity while at the same time picking up tissue and pushing it forward. And because of this fragmentation a vital organ hit is greatly increased. In fact, it would be near impossible not to hit an organ with a center torso hit. ES makes the claim that it throws the central nervous system into shock with the damage thereby shutting it down. And that is precisely what is wanted.
The round is designed to dissipate its energy in the first 5 to 9 inches. The FBI look for something that will penetrate 12 or more inches. But you know what? From the front of my chest to the back is only like 8 inches. 12 inches of penetration means there is 4 left for someone else. Not only that, it also means that 1/3rd of the energy didn't transfer to the target. That reduces my 600lbs of force to ~400.
I don't see myself in a situation like the FBI where I might need to shoot through walls, homes and cars. The vast majority of self-defense encounters are against assailants who are facing you and who are not wearing armor nor are hiding behind cover. I want as much as possible to go into that target with little to no over-penetration and I feel this round will do that for me.
They have some video on their site of various rounds fired into ballistic gel. The .50 BMG is so total overkill but it is interesting to watch the gel blow up.