The .45 S-ATPS is a semi automatic pistol, chambered in .45 ACP. It features good customizability, being able to mount a sight and an under barrel rail.
The .45 ACP is a round that works both ways, it has good stopping power at close range, this makes it quite effective against zombies, the large diameter helping disipate large amounts of energy to the target. Also the round is quite commonly used in the USA so running out of ammunition should not be much of a problem. The reason I don't like the round is that the low velocity and large diameter make bullet drop a big problem. Also the accuracy is not good because of this, making the S-ATPS rather useless in longer range scenarios.
The S-ATPS is reasonably customizable, allowing one to mount an optic and a small underbarrel accessory. I would mount a small red dot sight on the top rail, as I don't see anyone using the pistol at any significant range. I might just leave it with the irons as a red dot on the slide would get damaged quickly.
The innovative gas operation system is a hinderance in most ways. When you are out zombie hunting the last thing you want is a jammed backup weapon. The gas operation system is liable to get dirt and other muck in it, which is just going to jam up the whole thing like M16s did in Vietnam. The system would need to be cleaned regularly, which is not very useful.
Overall the S-ATPS is not a great choice for those looking for a good secondary, the disadvantages of range and reliability are too much to sacrifice for short range stopping power and custimizability.