I never used one. For me it's close to cheating.
Cheating implies breaking the rules, and these are part of the game.
Plus they aren't exactly "press button to win", they can give you a little bit of an edge but they're not exactly boss killers on their own or anything.
It's like having potions vs not having them.
Bosses can be done without them but having them gives you an advantage.
I still have trouble with bosses even when I do use them.
Used a Sky's Rage on Cerberus, still almost died when it wore off before I could defeat the boss.
It didn't wear off when Cerberus had a sliver of health left either, it's was like a good 25-35% left.
Didn't really even phase Typhon either so I still had a really hard time.