I find the power word spells, including kill, to be fine as-is. Haven't gotten one of my campaigns high enough level to evaluate if my players will be utilizing them, but I do know that I intend to have NPCs utilize them to present a type of threat which "illustrates" the dangers of such high-level magic, and if I were to sit down today and be playing a high level wizard with at least 3 9th level spells in my book, one of them would be power word kill and I'd have it (and the other two) prepared.As to the usage... I don't think it is particularly difficult to use, as I am a DM that runs full-transparency so that players do not waste resources like spell slots by casting them with no idea of the odds of working, but I do see the spell as being situational - like every other spell.
While I don't exactly agree with Treantmonk's conclusion of "maybe your best bet is just to cause 100hp of damage." I do understand why it has been made; at the level of play when this spell becomes an option, the party has numerous methods of dealing fairly large amounts of damage in a short period of time, so another 100 hp of damage is probably only a round or two of lower-resource-cost attacks.
Except for when it isn't, which is where this situational spell becomes worth casting: if the target has potent defense against the party's forms of attack, whether it is some regenerative ability, legendary resistance or otherwise potent saves, a high enough armor class to reduce the effectiveness of attacks, particular immunities or resistances, really anything that significantly slows down the party's damage-dealing rate, or even a matter of circumstantial time-restriction (i.e. something bad happens if the fight continues 1 or 2 rounds longer), or of the target having potent attacks that the party will be greatly advantaged by preventing - then the ability of this spell to say "You die, right now, no save, no chance to miss, no recourse but coming back from death." is suddenly no longer unimpressive and seemingly not a solid choice for expending your very likely only 9th level spell slot on using.
Is that sort of situation going to happen often? Probably not - but then, I don't think situations in which casting a 9th level spell of any sort makes the most sense come up all that often in the first place, so being particularly situational doesn't register as a "con" for me.
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