Gizmodo explains that these rounded corners are easier on the eyes and easier for the brain to process because the fovea of your eye can process circles faster than squares. So apparently you will prefer the circular buttons because they don't make you 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.
Compare the images below and let us know what you think!
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