It is[0]. Duff's device, whether you like it or hate it, was a performance optimization at least, saying "idx[arr]" instead of "arr[idx]" does not help in any way and will make some people angry or sad.

Contestants in the International Obfuscated C Code Contest will (and probably already have) find delightful ways to use and abuse this syntactical curiosity.

[0] My one-track mind always wants to jump to pirate jokes.

EDIT: If you like to explore this area, I highly recommend "Expert C Programming - Deep C Secrets" (get the pun? There's a fish on the cover...) by Peter Van Der Linden. It's pretty old (it talks about DOS memory models, among other things), but it is a fun deep dive into the murky corners of C. I wish there was an updated version and/or books covering other languages in that style.

> Contestants in the International Obfuscated C Code Contest will (and probably already have) find delightful ways to use and abuse this syntactical curiosity.

The OCCC is where I learned about this in the first place!

Surely it counts as useful if it could be used to confuse The Enemy. ;)