The partially unusable screen acts nicely as an easy to hit back button offering context about where you'll go back to. The spatial organization and animation showing the menu adds even more context, making this way of implementing a 'global' menu a pretty clever design all in all.

My only gripe with it that it might make it too easy to bring the complexity of big menus of the desktop to mobile device. As programmers we're (hopefully) all aware about the power and dangers of using globals.