I got my Xcode into a non-functional state when I can no longer start a search: once I press ⌘+F, Xcode hangs with a spinning pinwheel, and stops responding. Once
This is the officially recommended way to delta Xcode 5's preferences, type in Terminal.app:
defaults delete com.apple.dt.Xcode
That should restore Xcode to the state of its first launch.
(for older versions of Xcode the command was defaults delete com.apple.Xcode, i.e. without the dt in the middle).
Reset all XCode settings from command line by running following commands then restart XCode:
defaults delete com.apple.dt.Xcode
rm -rf $HOME/Library/Application Support/Developer/Shared/Xcode
rm -rf $HOME/Library/Saved\ Application\ State/com.apple.dt.Xcode.savedState
rm -rf $HOME/Library/Preferences/com.apple.dt.Xcode.*