I've been using the xmonad window manager, which is roughly 1000 lines. While not a vastly complex system, it's non-trivial, especially solving things like gui interaction in a side-effect free world. I believe they had to use some sort of zipper structure for the mouse detection to overcome things that are easy in other languages.
I wouldn't say zippers are really "overcoming" anything, they're very nice and elegant data structures, particularly because any data type has an associated zipper form.
