It rarely gets mentioned - but peep/aperture sights just don't work for some people. I can hit center target all day, get groups between 2-3" at 100 yards, with tangent sights (post and notch) and a good rifle, without straining myself.
Put an aperture sight in front of me? I'm lucky to hit the safety berm. I've tried, repeatedly, with different techniques, to get myself to be able to hit targets with peep or aperture sights. I've succeeded in being able to take a very accurate rifle, and hit a pop tin at 25 yards, about half the time.
And it really annoys me, because there are some great rifles out there with peep sights, and the "conventional wisdom" is that you should as accurate or more accurate with peep/aperture, and I suspect most people are, but for me it's just a mess of jangled visual cues that don't add up.
I'm also one of those people who gets an instant migraine at 3D movies - I have a suspicion that the two things are related.