One thing I like about iOS 5 is the rounded toggle button that appears within the settings. I cringe when I look at the rectangular buttons of iOS 4. This seems crazy, but there's a scientific reason for this preference.
Rounded corners are easy on the eyes and easy for the brain to process. That's because the fovea of your eye can process circles at a rate much faster than squares. You prefer circular objects because your eyes and brain don't work as hard.
Rounded corners also point inward and focus your eyes on the content inside an object. A squared edge is the exact opposite — it focuses your eyes on the stuff outside of the object. When you look at the rectangular buttons of iOS 4, you have to work harder to shift your gaze from the outside to the inside of the button. In iOS 5, there's little work involved. You look at the round button, your eyes focus inward and you make your selection with ease. [UX Movement].