I bought a used XD-9 service with the XS Big Dot sights already installed. I recently moved them to my daily carry XD Sub.
Impressions-
VERY fast sight picture. I mean, basically instant sight picture. It's an awesome sight for rapid, close range shooting. I have never used a pistol sight which works as well for close range defensive shooting.
However, once you get out beyond 15 yards, the huge front dot obscures the target and makes precise shooting very difficult. This was a big problem when trying shooting normal steel targets in IDPA matches as the sight completely obscured the plates at 15+ yards. I totally blew a stage due to this problem. After the match was over, I reshot the plate stage with the wife's conventionally sighted XD-SC and had no problems dropping the plates rapidly.
So now my SC has the Big Dots on it, and the service has fiber optics
Long story short

- I think they are some of the best sights on the market for a defensive carry weapon. Within the magic 21 ft, they are awesome and they are ok for shooting at human sized targets out to 25 yards.
If you want to also do competition shooting or precision target shooting, I suggest looking at a more conventional sighting system.