I use the medium one at the range, it helps with heavy calibers.
Not hunting, it´s stiff and uncomfortable.
I agree with
@Nyati on this. The PAST Magnum pad works, but uncomfortable, for me, it rides up on the shoulder, and I have to readjust every couple of shots on the range. Also, as others stated, it is thick and affects the LOP, on some rifles affect the gun mount.
For a strap on pad, Limbsaver makes one thinner than the PAST, works as well or better. I didn't know about these, but my shooting buddy bought one for me and I really like it, my go to.
Browning also makes a pad, to be sewn into your shooting jacket, vest, or shirt called the Reactar.
I have a Reactar pad sewn into my thin, cotton shooting jacket, works almost as well as the Limbsaver strap on and comfortable. I also use it when bird hunting with my 12-gauge double. Gun mount is easy.
Definitely use a shooting pad for the bigger bores. When I moved up from .375 H&H to 416 Rigby, a shooting pad became essential equipment. I could go from shooting 4-5 rounds proficiently, to up to 20 rounds. That's just me, rifle fit, stock dimensions, powder charge, form all make a difference. A shooting pad always improved proficiency. And reduced the tendency to flinch.