In my very firmly held opinion, a totally worthless band-aid. In an effort to cure my old wandering, squirrely Dodge B ('99/'00 RT D190V), I installed one, carefully & per instructions. Didn't make one whit of difference.
But less than 500 miles later, just after driving the curvy west end of the Blue Ridge Parkway, the power steering pump failed, totally & without warning. Thank whatever powers may be, I was turning around in a parking lot at the time. Fluid was full & clean. Can I prove a connection? No, unfortunately. I'd be rich if I could.
But if I owned a honey wagon that badly needed steering help and S-T-P was the only thing that would fit, I wouldn't put one of those POS's on it.
Jim, "Mo' coffee!"
(on edit: Yep, my experience was with 'Safe-T-Plus', the spring-centered steering shock in one housing)