Much to my surprise, the warlock Felguard build is actually a viable leveling tree. Before level 50, however, I doubt there's a very good reason to be in the demo tree, and if you're planning on using the voidwalker for anything other than very specific overleveled-named-elite mob encounters, you're doing it wrong.
As soon as you have the succubus, you use the succubus for leveling (and grinding in general). Then generally means you want a concentration talent. Affliction, unquestionably, is simply better than any other warlock tree, and most other trees in general, at killing multiple targets at once. And Dark Pact is quite possibly the best grinding talent in the game--it turns out that leveling is about sustainability and consistency of damage, rather than absolute DPS, and giving your character access to ~400mp/5@70 does wonders for sustainability.
WorldofWarcraft.com -> Info -> Classes -> Warlock -> Talent Calculator is the basis of a good affliction leveling spec. UA doesn't end up being that useful because it's not instant-cast, but it has potential.
If you want to go felguard when it becomes available, it's an option. It has similar damage, and similar sustainabiltiy because a lot of your damage isn't coming from your own mana supply. Personally I don't like the playstyle at all, and I still think that affliction does better at fighting multiple mobs which gives it an edge in hardcore grinding, but also requires better gear. Levels 40-50 are affliction.