To borrow a phrase from MB, XS sights are "mission-specific"....they (like the original express sight) are made for hitting large targets at close range very quickly. They're not precision sights....nor are they long-range sights....but they do what they're meant to do very well. I once owned a Glock 17 with a set of XS Big Dot Tritium sights that I used for USPSA matches in my area. They were great for the close-up targets; but, when the 50yd. targets came up, they didn't work so well. I've heard that some people have had luck using a rear adjustible sight (like Millett) and sighting in for a 6 o'clock hold at 50yds, but I sold that Glock before I could try it.
It all depends on what your "mission" is.....