I put the Big Dots on two of my Glocks, a 26 and a 27 and had them on for the better part of a year.
I found them plenty accurate for close shooting. However, I found when shooting really close and fast I didn't use the sights at all. Shooting further back, I found the XS worked fine, but noticed I had to really concentrate to keep the big dot aligned over the post in the shallow "v" rear sight. I found that my shots would often drift left or right.
To keep things lined up and shoot with more precision, I put in a OEM Glock "u" steel rear sight. This corrected the lateral drift, and works well. The standard dot would probably work even better. After some time with this configuration, I found that the big dot covered too much of the target and replaced it with a OEM steel Glock front. This combination works the best for me, allowing accurate shots and quick alignment. Not very dramatic or custom as its factory equiptment. Too bad the steel sights are not put on the pistols from the get-go!
That's my expierence, I loved the XS - but over time found them less than stellar if I really wanted more than 10 yard accuracy.