I've never used flags. Probably because I watched one of Snail's videos a few years ago where he drummed it into me not to. My understanding is that it's not worth using flags since they can't do anything that Alterable Values can't already do, but come with several disadvantages (mainly the inability to name them, which makes for messy code that is easy to get lost in down the track).
But are there some legitimately good uses for them? How/when do you guys use them?