Yeah, the curse of weakness works pretty well with the Succubus. There are occassions where you'll have to funnel life to her, but she far outdamages the Voidwalker so it balances out. Specially if you have good first aid skills and can just heal yourself while the mob is concentrated on her.
There are a few occasions I've found though, where I have to switch to Ol Big Blue. I was lvl 35 at the time, fighting lvl 37 elite Ogres solo, and the succubus just couldn't hold aggro with the amount I had to heal her. So in this case the Voidwalker did the trick. It worked ok because I found a safe place to pull to, and could take the time to kill the mob. The big advantage with Suc is that you can take down mobs pretty quickly, and if you're fighting humanoids and get an add, she can seduce it, while you kill the other one. It takes some practice to get the balance right with her, but she actually is really effective in most, but not all, situations.