I haven't used them, but don't like the idea. On a handgun, both the rear sight AND front sight are far from your eye. The rear aperture seems like it would simply block your target, even if, and perhaps especially if your sights are aligned properly.
On a rifle, the rear aperture is close enough to your eye that the aperture becomes blurred, and "seeing around it" isn't really an issue.
For further understanding of the concept I'm dancing around not very eloquently, see "parallax".
