Save or dies are generally unfun in my opinion. When the only options are accomplish nothing or win, based off of a random number generater, it steals the sense of accomplishment that I get from succeeding in the challenge. One shotting mooks and much lower level PCs is different as you expect to be able to take care of something weak with minimal effort, either through spells or an axe to the head. However, you don't need save or dies to do that, any significant amount of damage accomplishes the same result.
Basically I agree Grumpy above me. Auto kill spells just aren't satisfying even if I am the one using them.
