I leveled as Demonology on both my Warlocks and it was pretty great, although this was back when Harvest Life was still cool.
Before you decide on a spec, you should pick your preference:
Drain tanking = Affliction
Burst damage = Destruction
Badass = Demonology
I may be biased.
Destruction is pretty great for your standard leveling, because you don't have to worry about mana regen (you regenerate mana super fast, but you lose Life Tap), and because you get Ember Tap, which allows you to spend Embers on self-healing. It's a fast-paced style where you try to blow **** up before it gets to chew on your face. If you use a Voidwalker/-lord, it's almost too easy.
Affliction isn't really all that great for leveling, unless you can chain pulls. It excels at drain-tanking due to how the Soulburn mechanic works with Drain Life, though really any spec can do that these days, if you get the Harvest Life talent and the Drain Life glyph. Style is kinda like Shadow Priest, except you lack burst damage.
Demonology is just badass. Yes, it's a bit slow, but it's a trade-off for getting the best of both the other specs. You're not quite as good at applying damage over time as Affliction nor are you as good at bursting stuff down as Destruction, but you get utility in return. With the Demon Hunting glyph, you get the ability to tell your Voidwalker to sod off. I liked this style a lot. You can also just stay squishy and pull out your Fel-/Wrathguard and have him go nuts.
My main reason for enjoying Demonology while leveling was Demonic Leap. Not only does it give you a pseudo-blink, it also enables you to jump off cliffs and not die, if you get the minor glyph for it. The style is more centered around managing your mana, though, as you need to Life Tap quite a lot in dungeons, but you'll be pretty much invincible while questing. You get the most beefed up demons of them all.
Note: Until you get your Felguard at level 42, use the Felhunter. It has, by far, the best single target DPS of them all. The Imp only wins if you need to switch targets a lot, and only if you're at range. If you use the Demon Hunting glyph with your Felhunter, you will be fighting alongside your demon, so it's a nonissue.
Demonology is slow early on, but if you get the Demon Hunting glyph, it'll be lots of fun (if you like the melee style).
For a more in-depth guide, I'd recommend giving this a look: http://www.icy-veins.com/warlock-wow-leveling-guide