One of my first combinations was Warlock + Priest, on two boxes without any broadcast software.
The priest was Disc/Holy mix, and the warlock was Dest/Demo mix.
It worked fairly well.

The mage will, out of combat, heal as well as the Priest, as far as Lifetap out of combat heals go.
Just have the two toons duel, and a polymorph will top you off every quickly.
You can break the polymorph with a rank 1 spell, which will do next to no damage.

I'd still go with the Priest over a Mage.
You have the option for in-combat heals.
Fort and Spirit are a stronger buff option then Intellect.
If you want Intellect, a Felpuppy will eventually get you that anyway.
A Demo Warlock with Improved Health Funnel can solo most 5-man group quests of their level.
I'd imagine a priest would be better heals.

Until the Felguard, affliction is a very fast spec to level as a Warlock.
You have instant cast Agony and Corruption. The priest adds SW: Pain and Devouring Plague (30sec cd).
Howl of Terror is eventually instant cast. Psychic Scream gives you a second fear.
You can tap a lot of things and keep running, just stop to loot if you wish.
Pet of choice is imp, on passive if you take its mana.
Switch to succubus with Lash disabled once you get her as a pet; she's about as effective without the mana move and you can drain her mana too.

Affliction will be faster then a Felguard spec, if you pull masses of mobs of your level to 3-4 levels below you.
Demo will allow you to take much more difficult opponents down. Only a hunter will rival your ability at this.