I have one on each of my three 870's (two 12ga and one 20 ga) and one on my Remington 700 .30-06, and I think it works great. It really reduced the felt recoil a lot.
As for the fit of the pad to the stock- 3 of my guns have the older version stocks on them and they are a little tall and skinny as the other person mentioned... the one I have with the newer style stock seemed to fit better but still not 100% perfect, but I can handle that since it saves a bunch of wear and tear on my shoulder.
Be careful on what type you order though- they have 4 SuperCell pads. Shotgun with wood stock, shotgun with synthetic, rifle with wood and rifle with synthetic. I've seen reviews on some sites saying that the pad didn't fit at all... not sure if they bought the wrong one for their stock or what happened, but all of mine fit- just not 100% flush.