I had an affliction lock on my mixed team, for quite a while. Against trash and quest mobs, the DPS was pretty pathetic. But against bosses, the warlock was the highest DPS on the team. That warlock experimented with Destro, becoming highest DPS on trash, but never reaching the Affliction level on bosses. I've since moved the lock to my 3x Elemental & 1x Protection team, for the 13% spell power buff and run her as Demonology now that she has enough SP for the lock buff to provide more SP then the Totem of Wrath.

The 4x Warlock & 1x Priest team (my only horde team) is sitting at 21st or 22nd. Pretty low priority at the moment, but they'll end up Affliction. Still undecided on 5x Warlocks or 4x and a Disc Priest.