Thats a very open question. There are times I have used it in the past, but not lately.
vs. Mages/healers - I wouldn't bother...a mage at or around the same lvl as you has too much MP to drain all of it. I think you would be out of mp just from casting it over and over.
vs Archers - It's a good skill to use back in C5 cause you could sleep > aura sink > wait a few secs > and repeat. There MP would be gone after about the 3rd or 4th time it lands. But with the new updates (if I remember right) aura sink now wakes up slept targets.
If your doing 1vs1 or small group pvp, sleep and stun are your two best skills. If you stun your target just LD them to death. With my WC'er I usually look to sleep and stun as fast as I can so the rest of the party can run through and pick ppl off one by one.
But there are many different ways to setup and play your WC'er. Which is why I like the class so much. Personaly I wouldn't bother with that skill untill you have the sp to get most if not all your other skills.