I used one of these for the implementation of Siphash in the Linux kernel, if you're interested in a real world example: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...
It's unusual because in structured languages you expect the lexical structure to be entirely hierarchical (you wouldn't have an if block that somehow overlapped with a for-loop, for instance). Duff's device breaks that assumption because it uses one control structure (the switch) to jump into the middle of a second control structure (the loop), and lexically, the switch is both inside the loop and outside of it.
This is why lots of people when they first see it don't think it is even syntactically valid. A regular switch with fall-through doesn't violate the "hierarchical lexical structure" assumption in that way.