I sometimes use it when using the device as a tablet, but I mostly use it with the Type Cover in laptop mode. And here it seems a bit redundant, especially when on the desktop as it means I can 4 different windows keys at once (Bottom left corner of Desktop, in the Charms Bar, on the device itself, and on the keyboard).
Ideally I would have it as a softkey (like the Nexus 7 home key) that can move around the screen as you rotate so that it's always at the bottom, or disappear entirely when the keyboard is attached to provide more screen real estate.