I can't speak to the 20ga. but I have found their 12ga. brothers to be devastating. The only problem I could see with them is that they over-penetrate, even at 100yds. I have shot deer with them from 40yds to about 110yds and I've never had one that didn't go all the way through the critter. Granted, most of the shots were (more-or-less) side shots, but I've had them blow out both shoulders ("note to self: aim further back") and just keep going.
I did have one doe I shot a couple years ago head-on. I shot her to the right of center in the chest as she was facing me at about 75yds. The slug broke the shoulder into several pieces, took out most of the ribs on that side, tore through the abdomen, and appeared to have exited between her thighs (never found the slug, did find two bruises on the inner-thighs). I didn't pick though the gut-pile looking for the slug, I admit, but it really tore her up and put her down hard. I don't think I'll ever shoot one in the front-chest like that again. With the accuracy of the CS in my USHD, I will be as far ahead to aim for the head within 80yds...
