I started my warlock off with Demonology mostly cause at the time I wanted a character with a pocket tank. By the time I got to twenty I switched to Affliction and never looked back.
The higher I got, the better it became, I usually just run around and cast Corruption + Curse of Agony, Tab, Corruption + Curse of Agony, Tab, repeat repeat repeat and never stop running. Every few mobs hit life tap and you'll never run out of mana and never die and mobs die constantly. Makes kill quests delicious.
I hiiiighly suggest Affliction just for the ludicrous slaying of hordes of terrified creatures.