While leveling...
Pretty much anything will work.
Mobs die pretty easily and all.

Once you are maximum level and gearing up.
All three spec's are viable.

Affliction is probably the worst (for a boxer, maybe the best for a solo toon), in that mobs will likely die before the dots finish ticking, but you've spent the mana on say 3 casts per warlock.
Also, if you're going to be pvping on them, Affliction gives them time to heal.
But once you get to a certain spell power point, this will be a very fun spec.
Affliction has passive heals from Siphon Life and Haunt, which is quite nice.
Plus toys like the instant fear, or Unstable Affliction as Dot Protection, Curse of Exhaustion etc.

Demonology gets you Soul Link, which is increased survivability.
The Felguard is a very nice pet, the charge Interrupt is strong on round robin.
You get higher raw spell power then either of the other specs, Seed of Corruption is pretty strong with this spec.
The Demon Form with Immolation Aura can be pretty sick.

Destruction plays like a mage.
Immolate -- Conflagrate.
Chaos Bolt.
Shadowfury on round-robin for the stun can be nice.
This is probably the most straight forward spec to turn into a click macro.
And is also the least wasted mana, with stronger personal dps then the Demo Lock.

My locks will be Affliction.
But have not decided between Demonology or Destruction for the gearing up stage.
Both have merits and it will come down to which is more fun.