I haven't activated my slave accounts in cata still, but at lvl 80 5 warlocks were really fun in pvp and a complete mass destruction squad.

I ran them all as demo spec with felguard. The demonic empowerment making felguard unfearable and immune to cc made usually good work of the other team healer. Then pop demon form + immolation aura and start spamming seed of corruption. In a couple of seconds everything with red name usually ends dead.

Immolation aura x 5 does HUGE damage. and now with the new talents and the option to hellfire while moving I am thinking that a warlock squad could be renamed to suicide pyromaniac squad. Thinking of this kind of macro...
/castsequence demon form, charge (in the middle of the zerg), immolation aura, hellfire, /dance, /lol