I recently pulled Nightmare from CiTW and have been playing around with it.
On eNorm it's pretty wicked. When playing on eHard, because of higher saves and epic ward, it seems to be not as good as weapons with more DPS (khopesh).
On eElite, I think it works pretty well. Since the mobs have so many hit points... the life stealing (neg levels) proc makes up for the less DPS.
I'm wondering what others think about this weapon. Is it worth using on EE content... or should I just go with a straight DPS weapon?