My 5 Warlocks are currently L63 and managed every dungeon up until Ramparts, last boss, with ease (we don't mention the immune-to-magic mobs and silencing mobs in scholo, they have been wiped from our memory).
The last boss in Ramparts just does too much fire damage to our big clump of clothies before landing.

They're all deep demonology specced, with Felguards for outside the instance/PVP and Voidies for inside.
They really are great fun... setting 5 Felguards on someone and following up with up the 15 dots is great... everyone melts

However, I think I would definitely have had an easier time with 4 Warlocks and 1 Priest, and if I get really stuck in Outlands dungeons, I'll swap in my 70 Priest and try that configuration.

By comparison, my 5 elemental shammies are even more powerful and are absolutely eating up all the dungeons levels earlier than the Warlocks did