As long as all your tiles support the appearance, the effect is really really slick. By contrast, iOS 7’s parallax wallpapers feels gimmicky and cheap. I’m really happy to see Microsoft experimenting here.
The importance of wallpaper customisation cannot be understated. Users love setting images of their pets or loved ones as their background. A common iPhone behaviour is for users to put all their apps into one folder, just to maximise the amount of wallpaper they can see.
Unfortunately, apps like Facebook or Twitter (which currently stick to their own branding regardless of user options) spoil the illusion somewhat. It’s particularly disorientating when you scroll; there is a visual clash between the static backgrounds and the one that tracks the panning.