Well...nothing yet to be honest. I was thinking on using it on a Shiny Charm legendary hunt, but I still haven't imported my Pokémon yet to complete the dex.
I very rarely use Quick Balls on legendaries though, as I usually spend the first turn doing a speed check. If it doesn't make the mark, I reset immediately. The only time I captured one was with the Latios, and it was lucky that it was a top speed.