I leveled my lock (currently 73, been shelved since I was leveling my Pally to 80) as Demo for Felguard up until Outlands, specifically HFP. I found that, especially in that one Orc camp area, I was constantly getting swarmed and I was often leaving combat practically dead and oom... my Felguard being in the same situation. I switched to deep Affliction after that and haven't turned back since. I kill so fast and typically end fights with even more health and mana than when I was Demo. As someone else said I use the Voidy when I need to take on an elite or something, for the initial aggro hold, otherwise I usually drain tank with a Felhunter.