[3.5] How balanced are Thri-Keen?

[Sstoopidtallkid]

But if you spend a standard action and a spell slot to change their saves from 50% chance of success to 35% chance of success that's really similar to casting the spell twice when they have a 50% chance.

It's better in some ways, like it might be a lower spell level than the save or X, it might also be worse, in that they could be immune to it, or you could miss the RTA (really only for high touch AC monsters, which are few) Or you could hit them, but they roll high on their one roll.

Enervation + save spell is good, but by no means actually fits the "casters kill everything in one round" complaint.

I meant Enervation+death. The spell applies penalties to everything, not just saves, and works well with metamagic.