From my Night Sight Review...
XS Big Dot
sights are high quality combat sights, designed for SHTF situations. Place that big white dot on center mass, and if your trigger control is good, you will hit your target, out to 30-feet or better. The night sight picture is pleasing and easily picked up. These are a specialty sight, and many will find them wanting for range use; conversely, others will master them for target use too. More than any sight I know of, people like them or they don't.