I Use Them...
...on the pair of S&W 640-1's I carry on a daily basis. On these revolvers, I only have the Big Dot with the tritium insert on the front. Out to intermediate ranges, such as ten yards, I simply seat the dot in the groove and the bullet usually impacts about where I would see if I could look through the tritium dot in the center of the front sight. A few years ago I fired a pair of "qualifications" with these revolvers for a Nevada CFP renewal and, as I recall, had one round just barely over the line into the nine-ring one each "qualification," one of which was shot right-handed and one left-handed; the remaining rounds were all in the ten-ring. I don't recall if the course of fire extended to seven or ten yards.
More recently, one of my students brought a pair of Glock 26's, with the Big Dots paired with the shallow-V express rear sights. I tried one of them and found it comparable and (for me) comfortable.
Many people, such as myself, like these sights for defensive handguns as the Big Dot tends to intrude into the visual cone when your brain is more inclined to focus on the threat than on the front sight. Some older shooters find that the Big Dot is an adequate answer to presbyopia (age-related difficulty focusing on close objects). I don't imagine that I would ever want this system on a dedicated target pistol, such as a S&W Model 41. However, if I ever start carrying an autoloader that is not already fitted with more conventional night sights, I suspect that I'd go for the XS system, at least with the tritium dot up front - I have not had the chance to try the tritium bar below the center of the V of the rear sight after dark.
