I've been wearing one for 3 years now. It's actually the back piece to a motocross body armor suit. It has been a life saver. I have fallen a lot on my back and it has served me well. It's purpose (at least mine) is to actually spread out the impact through the back, rather than a direct point of impact.
In one case, I under rotated a frontflip (after catching my edge on the lip) and fell straight onto my back. I hit really hard, like 10 or so feet straight down directly flat on my spine. When I hit, I instantly had the wind knocked out of me. After getting up, my whole back was bruised and had a hard time moving my back. I got back up the lift and front flipped the jump properly as reinsurance that I could do one (so I wouldn't "fear" frontflips later on). When I got back home, my upper back was black and blue beyond belief. I was off skis for a week or so recovering.
Without that back piece, I'm pretty sure I would have been in a wheel chair right now. I don't ski without it. Even on simple groomer days or if i'm shredding 3 feet of powder.
I suggest getting one if anyone is interesting in progressing. It's quite a confidence booster.