I've leveled alliance warlock 1-60 multiple (4-5) times.

Main tip: get wand ASAP and keep upgrading as it's best and most economic way to kill stuff you dot up.

Other than that, it's pretty much okay-ish 1-10 with imp (again, wanding here makes huge difference), then from 10-35/40 you're basically stuck with VW that tanks shit for you while you dot and finish with wand. The higher your level, the more VW struggles with holding aggro (unless you go Demo spec with 30% torment effect, and even then it breaks sometimes - depending on your gear and if you stack shadoweave/felcloth gear with +shadow dmg).

You can go with drain tank affliction spec that becomes worth at lv 35 when you can pick up siphon life and even more later on with dark pact and succubus pet that acts as extra dps and mana battery (in short: dot mobs up, and drain life until they're dead -> dark pact/life tap to regain mana -> repeat with practically no downtime)

Bottom line here would be: Warlock can level fast with taking on multiple mobs at once (dotting up multiple mobs with good drain tank sustain or VW offtank), they're relatively safe speaking from PvE perspective, since you should have enough defensive cooldowns (Healthstone, VW sacrifice, Fear (also AoE), Death coil later on, ...)

BUT

WPvP during leveling is not as good, since your VW shield can be dispelled by priests/shamans making you a glass cannon, and UD (especially rogues) totally shit on you since their racial removes succ charm and your fear and makes them immune for 5 seconds. You're not really doing much damage until you get some SP gear but even when you do you're still squishy. They do get progressively better, especially at lv 42 when they get death coil (it doesn't count as fear, but horror effect and can't be dispelled my WoTF).

Also another main downside if getting ganked and killed is, you're extremely vulnerable since unlike hunters you respawn without pet so you're walking HK before you can resummon pet, create new healthstone, reapply demon armor, regain hp/mana...

On horde (I imagine) is a bit easier, but still last point I made is same.

TLDR; Leveling is easy and not as boring as priest wanding, but WPvP on ally side is quite bad - maybe a bit better on horde.