I have an XS Big Dot night sight on one of my Hi-Powers, oddly, it isn't the big dot I have a problem with but the rounded top. I am used to seeing the corners in crisp focus to verify I am focused on the front sight and just could never get used to the rounded top of the XS.
Also, for myself personally, I am not seeing any speed advantage over my Novak sights when I break out the timer. The flip side of that is I am not seeing the often reported downside to long range accuracy either. In fact, one of my best shots ever - a 50yd knock down of a popper from the running board of a Ford Explorer was made using a XS Big Dot.