I've got a 4x Warlock 1x Priest team.
Still rather low level.

I like the Priest because:
- Very mobile team (druid was the other mobile healer).
- Same race as the Warlocks.
- Gives the team rez.

I like a 5th Warlock because:
- More damage, more AoE.
- A 5 Demo Warlock team wreaked havoc and was unstoppable, in an AV this season.
- Lots of heals from Drains or Haunt, without needing the Priest per se.
- Good survivability already too.

The team is 22nd or so.
Haven't played them in a while.
But they were quite fun; just started on other teams.