I have a Windows Phone. The Facebook app (disclaimer: I'm an employee) is quite nice despite missing some features I want. It is representative of the Metro "design language". In everyday use it doesn't feel like wasted space. It's hard to describe without using but the layout uses lots of vertical scrolling and the space to the left and right is usually a continuation of the current screent that shows related data (in the case of the Facebook app you swipe from newsfeed to a list of your photos then messages). This is very different to the static screens popular on iOS, and feels really nice to interact with.

The problem Microsoft faces is that the unique design of Metro excludes the possibility of a port of existing mobile apps. Apps need to be designed from scratch using native code - embedded web views won't cut it - which is expensive to do.