For me, Requiescat is slightly under Vorpal Blade. Vorpal Blade always performs nicely when it crits, and Requiescat is decent damage on a consistent basis.

That being said, Vorpal Blade (if you don't have CDC) is going to be your better option unless you're trying to bypass physical defenses or fighting elementals.

It has it's uses, but it's a very situational WS.