Inspired by this thread, I found one on Amazon at an agreeable price, $80ish. I mounted it on my single stack with a PSA Dagger Micro slide. This pistol has been to the range once. As I recall, it was a bit left of aim and I intend to drift the sight one day if I ever get a sight pusher. The ReadyDot went on and as viewed from behind, it favored the left side of the sights. At the range, it was dead on. The sights are off, the red dot is right on.
My assessment is that it is no more or less visible in low light than iron sights. If the lighting is too low to make out the sights, there is not enough light to "power" the dot. The dot covers most of a 4" bullseye at the 7yds I was shooting. I instinctively centered the dot on the available target and all the rounds landed in the center.
So, I have something on my slide that looks like a red dot so I can hang out with the cool kids. It never needs batteries and is always available if there is any illumination. I could see how this would aid a new or inexperience shooter just getting started. It doesn't help or hurt me any. Group sizes were the same either way.