I've been leveling demonology for my newest warlock (hit 45 last night). My last warlock (currently level 46 on a different server) was leveling entirely affliction.
Both that had their pluses. I feel a bit more in control as demonology; the voidwalker is a pretty darn good tank thus far, and if he dies I can simply summon another in .5 seconds. Thanks to Sacrifice no longer killing VW, I have a very powerful shield on a minute cooldown for next to no cost. My repair bills are much lower, since I'm not drain-tanking, and my killing speed seems comparable. Affliction was fun for AoEing stuff, but demonology can do it too; gather a bunch of mobs, dismount, pop demonic Empowerment, AoE taunt all the enemies, put Sacrifice on and Rain of Fire (or Hellfire) all of the enemies. It isn't pretty, but it is effective and can be done every couple of minutes.
I think the make-or-break point will be level 50, when I get my Felguard. I am SO looking forward to that. A pet that tanks almost as well as Voidwalker, but does like 3-4x the damage? YES PLEASE! I will miss my insta-shield, but I think he'll be worth it.
Affliction was fine, but I felt less safe and couldn't handle adds as well. I'm not sure if the leveling speed is much different between the two specs.