The Apple calculator in iOS and OS X have remained pretty much unchanged since both were released. They work fine, but the Numerical calculator for iOS (free for a limited time) takes cues from the iOS 7 design language and adds some features that are terrific, but might take some getting used to.
The app bills itself as a "Calculator without Equal' and that is literally true -- there is no equals button. You enter your numbers, like 2+2 and 4 appears.
As you work, Numerical creates a history of your calculations, and there is an undo key that lets you delete your last entries. Many actions are done using swipes, which work for undo, redo and saving an answer. The app also supports the clipboard.