As arcon says, you shouldn't be using exenterator inside abyssea at all.

Outside abyssea, Exenterator is clearly better than Evisceration, but that doesn't say a lot because Dancing Edge is generally better than Evisc as well. The real question is if you should get exenterator to use instead of DE on the outside. It is definitely better, but not by as much as it is better than evisceration. Maybe the people I've seen just suck, but I generally see Exenterator do about 10-20% more damage than my own DEs do, and I don't exactly have the best DE gear in the world.