I actually think of it as a better tool for bodybuilding than for raw powerlifting.
In terms of powerlifting, the hardest part of a heavy bench press for most people isn't the lockout, it's the middle part of the range of motion, and the slingshot helps you push through that part through initial momentum. So for me, it just trains the part of the movement I'm already good at.
In bodybuilding terms, the top part of the movement seems to be largely tricep-centric. So it at least seems to me that this would be a good overload for tricep development. That's at least what it's felt like the few times I've put a slingshot on.
And it obviously has strong carryover to geared lifting. That's mostly what I think it was designed for.