RE: What will you use / have you used your Master Ball for?
...I had to use mine... on a Phantump... It knew Curse and False Swipe didn't affect it so after two failed Luxury Balls it cursed twice leaving it with 1 HP (I nearly had a heart attack) I decided not to risk it and catch it with a Master Ball. It was unfortunate waste but it was either that or start a whole chain of Phantump again. The Phantump isn't very powerful, bad nature and wrong IVs, but I like to keep as a memory of my successful chains.
...I had to use mine... on a Phantump... It knew Curse and False Swipe didn't affect it so after two failed Luxury Balls it cursed twice leaving it with 1 HP (I nearly had a heart attack) I decided not to risk it and catch it with a Master Ball. It was unfortunate waste but it was either that or start a whole chain of Phantump again. The Phantump isn't very powerful, bad nature and wrong IVs, but I like to keep as a memory of my successful chains.