I spec'd into Felguard/Siphon Life for leveling purposes. Everyone laughs at me, but I am great at casting my 3 instants and running around in circles staying at full health killing 8 mobs at once while felguard puts out 250 dps and kills them off.
Its the most fun I have grinding levels. I used to do it with Felhunter or Succubus, but with Felguard its even more teh awesome.
I've had people come up, stop, watch me, and send me tells afterward "Uh, how are you not dying?"
When you're regening health a tick and you're good at running in circles so you don't get constantly beat by melee, you really have a good bit of survivability.
The one place where this spec doesn't shine is vs large amounts of casters (but what Warlock spec does shine against 3+ casters?)
Against lower level mobs (I'm 72, now, I'm talking 68-69 mobs), I only use Siphon Life and Corruption.
Keeping soul link up also helps a lot.